Also in the Mad Wizard's Menagerie

d12
1. Glass dome containing mated pair of hovering jellyfish with human mouths, capable of startling mimicry, otherwise sing unfathomable chanteys in two-part harmony
2. Furry iguana-like reptile in refrigerated snow globe w/notepad and stylus, reads and writes several languages, over series of notes re: rescue from wizard makes claim of innate cartographic mastery
3. Disembodied brain in vat no longer remembers former identity, uses telepathy to request speedy termination
4. Still-defiant Yuri Gagarin in tattered spacesuit in lush habitarium, chained to wall, beaten half to death, depressed human female, latest attempted breeding partner, also present
5. Tank filled with tiny starfish-like creatures swarming around miniature rocket-shaped building project, beings consume gravel substrate of tank, extrude needed parts, still require human adrenal glands for fuel
6. Magical zero gravity sphere houses hive of void bees whose hypnotic movements enhance human intelligence if viewed for prolonged period
7. Cage, appointed stylishly, detains seemingly motionless body of world's oldest lich, conscious on couch, alert but no longer able to do anything but slowly desiccate
8. Just a cute little mouse in a terrarium that periodically vomits up a massive black pudding, automatic suction tubes swoop in to incarcerate puddings in huge sub-surface tank
9. The eldest dwarf, a literal living fossil almost entirely mineral now, sits unrestrained in open display, occasionally muttering history lessons in outrageous accent
10. Phosphorescent vapor-being, native to distant gas giant, can transmute matter on molecular level but requires fresh soul-stuff as nourishment
11. Completely friendly owlbear on long chain will forcibly (if lovingly) attempt beak-to-mouth feeding of passersby w/half-digested chow from nightmarish slough-trough
12. Goose that lays high explosive eggs

Weird Giant Generator

For use if inclined to generate weird giants. Roll once on each table, purée results into suitable menace.

Table One: Head
d12
1. Great bulgy eyes, ever googling, telescopic/microscopic vision, squints to perceive events in several dimensions/heavenly realms, offers occasional unsolicited reports
2. Huge bat ears, detects butterfly wing flapping on adjacent continent
3. Jaws work sideways, insect style, but otherwise human, speaks well, very inquisitive
4. Saber-toothed Neanderthal
5. Monocular, multiply horned, shark-like teeth
6. Like Max Schreck as Nosferatu
7. Quite stunning example of human-like beauty, fabulous hairdo, black eyes filled with stars
8. Tyrannosaurus head
9. Protruding rodent buck teeth, can chew through most anything at terrifying rate
10. Hideous mosquito head, harpoon-shaped proboscis, blood drain attack
11. Only burning eyes visible through shaggy caveman hairdo, incredible beard hosts complete ecosystem
12. Tremendous gaping but disturbingly toothless mouth swallows whole, emits stone-shattering hollers heard 'round the world

Table Two: Body type/size
d12
1. 30' in height, impossibly spindly as normal human stretched skyward by pitiless deities, astonishing strength and flexibility
2. 12' tall, nude w/smattering of woad, rippling with steely muscles from head to toe, veins writhe about visibly, savagely bites own hand to enter utterly berserk combat frenzy
3. Like a bipedal woolly mammoth w/opposable thumbs
4. Human proportions but 20' tall, clad in impeccably tailored finery on the bleeding edge of style
5. Bloated and swollen as if about to burst, 17' tall, naked but for custom skull codpiece
6. 36' of naked, green-glowing skeleton
7. 12' Frankensteinian w/huge heavy heeled boots, ill-fitting sport coat, stagger-lopes at surprising speed
8. 35' tall, 4 arms, 4 legs, roll for additional head
9. 18' clad all in found ironmongery fashioned of bits of armor, shields gathered from fallen foes
10. 99' of sickly, emaciated form draped in hospital gown stained grotesquely
11. Like a hairless 25' gorilla in a chainmail speedo
12. 40' tall Andre the Giant at peak of muttonchop period

Table Three: Additional oddity
d12
1. Survived crash landing of colossal UFO in polar reaches but brain damaged/gone feral
2. Literally thunderous footfall an innate magical special effect
3. 2d12 personalities vying for control
4. Cursed to wander from city to city begging for alms between destructive sprees
5. Involuntarily howls obscenities so you always know when he's coming
6. Terrified of small dogs for some reason, screams like proverbial school girl, assumes fetal position, whimpers pathetically
7. Unable to control lust for golden treasure the sight of which triggers childlike fascination, readily manipulable
8. Believes self to be incarnation of primal god of wrath, demands immediate fealty, works hard to maintain semi-permanent state of fury
9. Demi-material, 50% resistant to physical attacks, able to disintegrate/reintegrate in new location at will once daily
10. Automatic mind control: standard giant types, generally travels with motley crew (2d12 mixed giants)
11. Must constantly feed as shrew, carries huge reeking feedbag of various dead things
12. Swells to even grosser immensity if angered following brief, agonizing transformation period

Encounters on the Veldt

d12
1. Nomadic clan of humans w/20' extensible/retractable necks: live rough in the bush, armed with stone-tipped pikes, javelins, clad in red togas, sleep in shifts behind manned pike-wall, do not hunt but "borrow" blood and milk from giant mammals for sustenance
2. Lion-sized saber-toothed prairie dogs, construct massive house-mounds connected by miles of tunnels, engineer ambush tunnels to take prey, protect territory ferociously
3. Trade caravan hauling fruits of civilization, led by famous war hero/ex-dungeoneer, old drunken lout but still high level, giant wagons feature palisade walls bristling w/ballistae, 50 crossbowmen
4. Towering bison men tote titan-bone clubs, tend the herds, plot migratory patterns, speak common tongue haltingly but w/intensity of purpose
5. Bald, turf-eating titanotheres drool strong digestive acids, tend to stomp out bipeds as a preventative measure
6. The Colossal Pachyderm, immortal, all-knowing, trumpets out nightly prophecy, great ivory tusks said to molt every century or so, could be any time now
7. Speed-burrowing gopher-men pay a bounty in uncut diamonds for saber-toothed prairie dog heads
8. Hostile spirits w/huge grotesque heads scamper, dance atop the tall grass, call thunder from the clouds to set the rhythm, improvised lyrics viciously satirize adventurers
 9. Herd of tiny deer w/highly developed intellects only speak in numbers, automatically perform any calculation mentioned in presence, take great collective pride in eluding predators w/ruses of ever increasing complexity
10. Sessile but totally impervious Tortoise Lord, half-buried in mountain of sediment, issues great gouts of fire-breath capable of destroying targets several hexes away if convinced of its necessity
11. Ghost tyrannosaur stalks its ancient hunting grounds, delights in scaring hell out of intruders, capable of deadly demi-material manifestation only in proximity to clutch of fossilized eggs
12. Slaver birds, archaeopteryx-like but capable of hauling off human-sized creatures, take captives back to nest for period of terrible Skinnerian re-education then sell broken souls to pitiless traders

A Rumor of Dungeons

d12
1. Vast, yeti ghost-haunted cavern covered in obscene cave paintings allows access to Underworld via mystic doorway that must be constructed of enshrined cave bear skulls
2. Active ooze hive festers under the city, empties into nearby river/lake/sea, visited by famous sorcerer who departed atop perfectly obedient colossal pudding-steed
3. Death row convicts attempting escape tunnelled into unknown chamber, their dessicated corpses appeared on the courthouse steps at daybreak
4. Reasonably sane captain of the watch insists silver hatch appears beneath city wall only when in direct light of full moon, swears invisible things come out
5. Cult membership of the God of Plenty make fortnightly journey to underground complex in adjacent wilderness under the cover of night, never speak of it to outsiders
6. Unpleasant music emanates from farmer's recently dug well
7. Prince's pyramid project halted by lack of shipments from new quarry, trusted captain dispatched to investigate w/soldiery, not a peep
8. Sorcerer's griffin-borne aerial survey detects huge, ornately camouflaged mountain top chimney spewing forth weird gases
9. Rocket-shaped wizard's tower takes off, sub-surface floors left unattended, someone should at least strip the plumbing
10. Dragon moved in to proximate cavern not to plague the land in fulfillment of dire prophecy as is the prevailing wisdom, but rather to jealously guard secret entrance to antediluvian culture's treasure vaults
11. The Fetid Sewers of the Ancients drain into another world
12. War-gangs of ogres and mixed-humanoid lackeys loot frontier towns seemingly emboldened by smart attire, beautifully crafted garments/armor, chaotic evil chic, shields emblazoned with device of  the Tailor Below

Frozen in the Giant Block of Ice

d12
1. Severed head of monarch still believed to reign over nearby realm, gleaming crown included 
2. Indescribably beautiful corpse of human ancestor provides definitive proof of devolution, arms and armor of dazzling craftsmanship still in mint condish
3. Ancient vampire imprisoned by mythic heroes of the previous age: spent countless centuries perfecting delicious scheme of vengeance upon humanity, will immediately launch stage one the moment some idiot thaws her out
4. Huge, self-luminous apple: bestows god-like intelligence and/or fatal brain hemorrhage w/a single bite
5. Demigod still sleeping off epic bacchanal: encased in ice by trickster deity, incapacitated by equally epic hangover if freed
6. Massive diamond: actually a disembodied demon brain, subtle mind-control powers target the weak up to fifty feet away
7. Hapless missionaries of particularly obnoxious sect, thrown overboard in icy seas, divine intervention denied at the crucial moment, plenty of informative tracts, no treasure
8. Water elemental criminal mastermind: indistinguishable from surrounding ice but sorcerer patsy sticks out like sore thumb w/that bejewelled girdle, stylish low cut tunic reveals exceptionally hairy chest festooned w/ensorcelled amulets
9. Fully operational UFO: crash landed before able to transmit order to destroy the campaign world to home planet, pilots in stasis, can be magically/accidentally revived, lots of shiny, precious space metal parts
10. Intact corpse of alien sorcerer: bristles w/magical baubles, flash frozen while casting massive death spell, lethal energies released if ice block compromised
11. Demon-Fire Boomerang of Infinite Explosions: burns dimly w/in block, glowing sigil on ice surface flashes "Danger: Do Not Release!" in every conceivable language
12. Purple blob at block's center: shrunken, inert sun-eater called down from the void, bound by long-dead sorcerer to exploit latent potency, radiates magic of unprecedented magnitude, if freed will eat the sun

In the Ice Queen's Palace

d12
1. An army of rejected suitors, their frozen bodies positioned for use as furniture
2. Decorative liquid nitrogen indoor river feature w/spectacular faux waterfall effect
3. Free-roving sentient mists flash freeze intruders then interrogate telepathically
4. Fully bipedal polar bears in cobalt armor guard key doors and junctions, wield wicked war-forks, ferocious loyalty may be compromised by offer of fresh sea lion meat
5. Captivating icicle arrangement on ceiling shines and sparkles, save or stand fascinated as they suddenly launch toward the viewer in a single machine gun-like burst
6. Amphitheater with skating rink features performance by sword-shod troll of surprising grace, unruly audience of shaggy snow devils throw frozen vegetables
7. Personal igloo of queen's special advisor: The Lich of the North
8. Service zombies w/personal coal ovens installed in abdomen, utility belt featuring bag of coal, various pokers and little shovel on ring, flask of accelerant, bundle of wooden matches
9. Werewalrus in mortal guise as suave aristocrat w/terrific mustachio, stunning in layer after layer of the most opulent furs, requires the express written permission of The Ice Queen before feasting on human flesh
10. The Well of Absolute Zero
11. Colossal apparatus with giant radar-like dish aimed due south, storing psychic energy from ancient polar race in suspended animation until ready to unleash wave of preternatural cold upon Imperial Capital City
12. The Yeti Lama levitates in lotus position, invited to party ages ago, still hanging around, occasionally mutters koans, finds itself endlessly amusing

NOW WITH ORGANIZATION

Jeff Russell of the Blessings of the Dice Gods blog has, out of the immeasurable kindness of his heart, begun to create a linked index of Dungeon Dozen entries, broken down into helpful categories.
This is the kind of work that I'd rather throw myself down a flight of stairs than tackle, being both insanely busy right now and innately lazy, so I am in his debt.

Check it out here: http://blessingsofthedicegods.blogspot.com/p/dungeon-dozen-fan-index.html

The Giants Are Up To Something

d12
1. Depopulating and clear-cutting an elf forest, processing timber to construct the world's biggest party barn
2. Taking up surprise submarine river piracy through clever use of new snorkel technology
3. Several competing giant factions in arms race, kidnapping/enslaving dwarfs, stealing/stockpiling metals for bigger/better arms and armor, you should see the stone giant czar in his insanely ornate field plate
4. Entered symbiotic relationship with Underworld brain-eaters who call the shots, organizing into well-oiled machine, secluded fertile valley seized, pacified, ready to begin domesticating humans
5. Synthetic giant growth hormone derived from enzymes in titan bile, trade w/titan requires great volumes of gold, turns out giants willing to do anything to get even bigger
6. Building a colossal fortified bridge over strategic mountain pass for an unknown purpose
7. Thoughtless sorcerer's experimental ray of fecundity creates population explosion, many hungry mouths to feed, forays launched to strip surrounding countryside of wealth
8. Towering new god reveals itself spectacularly, normally indifferent giants suddenly explode into full on zealotry, begin work on vast temple, mandated holy site: Imperial Capitol City
9. Giant chief visited by ghost of ancient reptile man, initiates expedition to steamy jungle to begin process of taming the dinosaurs
10. Transforming secluded vale into world's biggest earthwork fortification, stockpiling foodstuffs in anticipation of massed hostile forces emanating from the Underworld
11. Premonitions of looming catastrophe drive mass giant migration to sea shore, whereupon they will somehow build their cyclopean escape-ark, possibly by lashing together the entirety of the Imperial Navy
12. Someone told them eating wizards makes you smarter

They Haunt the Caves

d12
1. Spirits of cavemen, kicked out of afterlife after hostile takeover of their Celestial Plains by more sophisticated modern deities, drum, chant, make a racket, subject slick city dwellers to obscene verbal abuse
2. Ghost of wizard, combusted spontaneously after unexpected arcane energy surge but suspects treachery, assumes adventurers are royal investigators and is quite prepared to give a full statement
3. Disembodied psyches of sentient slime creatures from the dawn of time ooze about mournfully, gurgle pathetically, hurl impotent threats in their untranslatable language of flatulence
4. Spelunker's soul remains stuck in tight crevice, thought he could free himself by lopping off limb, bled to death, still calls out for assistance
5. Spectral torchbearer eaten by giant lobster (long since deceased) swims around subterranean pool keeping watch over his bones and scaring the hell out of passersby
6. Ghosts of dishonored vikings, barred from entry to Valhalla, joylessly sing a mead-hall tune while trying to spruce up the place
7. Angry spirits of the earthbound dead cluster here, awaiting some kind of ghostly messiah to lead them on a tour of vengeance upon the living, standing room only
8. Spectre of UFO pilot refuses to acknowledge responsibility for own demise after disastrous attempt at cavern navigation, fragments of space-metal hull scattered about, cave walls, floors, ceiling show evidence of super-heated explosion
9. Phantasmal kings whose thrones were usurped by lusty barbarians over the ages cluster around a table-like rock talking about the old days and fomenting vengeance upon all rural, muscular, sword-wielding, sandal-clad conquerors
10. Apparition of hill giant, lured into eating a pie full of poison by thieves, rejected for afterlife service by wicked deity for supreme stupidity, mutters about tricky adventurers and things like "fool me once, shame on me, fool me twice..uh..me can't get fooled again..."
11. Gaggle of incorporeal executed intellectuals, subject of a imperial purge many years ago, seem perfectly content to chew the fat with anyone other than each other, ask leading questions, play devil's advocate just to sustain the conversation
12. Ghostly presence of murdered Dungeon Overlord more than willing to reveal location of former dominion and many of the secrets therein, if only the PCs would make a blood-pact to destroy his traitorous successor

Someone Tipped Off the Humanoids

d12
1. Humanoids staying w/cousins nearby, all portable wealth removed, monster husbandry expert stays back to release the watch puddings
2. Hired horde of lesser humanoids use guerrilla tactics to harass and deplete party while still en route to dungeon, paid upfront w/treasures formerly up for grabs on level one
3. Fake beards distributed, objects of open evil hidden, humanoids pretend to be clan of industrious dwarfs
4. Entrances festooned with tripwires, pressure plates, hastily covered pits and holes, barbed projectiles protrude from door slits, precariously balanced buckets totter over doorways, a huge pain in the ass
5. Main dungeon entrance rigged for lethal collapse, suicide trooper on duty, other entrances under heavy guard
6. Gas masks distributed (w/strategic exclusions, esp. that owlbear), level magically flooded with deadly vapors, tribe now permanently in debt to sorcerer from below
7. Wandering sorcerer employed to create hallucinatory terrain, misdirects adventurers to cave lair of terrible abomination they probably can't handle, sorcerer does terrific work, slaves haul all humanoid treasures back to tower
8. Sub-overlord notified, trio of medusae from unknown third level take up guard position formerly occupied by relatively wimpy humanoid watch
9. Dungeon entrance slick w/oil, sentries ordered to apply torches only after majority of party enters flame zone, 4 in 6 chance of sentry over-excitement
10. Humanoids massed by entrances, worked up into a lather by non-stop chanting and imbibing of fortifying beverages, ready to encircle and annihilate the party before they even reach the door
11. Entrance unguarded, rooms empty but for comfy chairs and tables with open bottles of wine (2 in 6 chance of poison/bottle), humanoids bide time in secret hidey-holes beneath flooring
12. Humanoids get enormous Chinese New Year-style dragon puppet out of moth balls, rig smoke bombs/pyrotechnics, prepare to bedazzle, scare hell out of adventurers while archers do their work from concealment