(Expanding on an idea from Noisms' Forge Monsters challenge.)
Time in the world above is measured by the progress of the moon, sun, and planets - an endless cycle of months and seasons and years. The zodiac divides time into increments and assigns each increment certain traits - an element, a mood, and most prominently, an animal. But while one born in the year of the Monkey may share the beast's cleverness and joy, the spirit of Monkey cares no more for dates and calendars than it does oceans or asphalt. The animal gives its name to the year, but that animal's god is not a god of time.
Below ground the stars are hidden and the seasons pass without notice. Time passes at the pace of shifting continents. Years are not named, but imposed. If the signs of the zodiac still bare the names of animals it is only because those creatures choose to make it so, because they find such ordered delineation useful.
The twelve beasts of the dark zodiac are prophets, sages, historians. Each is immortal - perhaps a god, perhaps just fantastically old. Naturally, all are completely mad, though some do well to conceal it.
Some find communication with the surface world suits their agendas, and have gained a certain notoriety. The Skull Hermit deals in cataclysms and the death of gods. The Wyvern of the Well can answer any question, but only once the question is asked. The Weaver considers itself an artist, beautifying the future's web of possibilities by violently cutting certain threads. Many-as-One, a rat king of unequaled population, reads the fortunes of cities by crawling through their entrails.
Others are less well known:
Grandmother Scorpion, five-tailed and bloated, can poison time itself, granting visions of one's past or future, prolonging life, or aging one to dust in an instant. Attending her is a cult of hideously withered elves, and she speaks through a mummified head and torso affixed to the end of her largest stinger. The price for her venom is a fresh elven sibyl, delivered within a year. To ensure the deal is kept she injects a thought-delayed poison that persists through magical healing, resurrection, reincarnation, and if all else fails, lineage.
Echo Serpent is an enormous, blind white rattlesnake that hunts by sonar like a bat. When its warning rattle echoes off cavern walls the echoes themselves are serpents, writhing shadows that strike and then vanish in a hiss of air. It is old as all Creation - not the tempting serpent of Eden, but born of the same divine word. If you tell the serpent a lie, it will whisper the lie back, and the lie will twist and coil in your brain until you tell it again. The thrice-told lie will deceive even gods, as long as telling it causes harm.
Still Carapace - a gnarled, chitinous old thing, not quite roach or trilobite - predates not just this multiverse but several others as well. Witness to the destruction of countless worlds (and countless more extinctions), Still Carapace is the best possible sage on all matters eschatological. Problem is, no one else can verify a damn thing the old bug says, to tell if the rituals he spits are meant to avoid yet another apocalypse, or cause a disaster only he knows how to survive.
Even more obscure beasts include a vampire bat that feeds on entropy (with anticoagulants in its bite that cause bleeding portal-wounds between worlds), a giant house centipede with antennae sensitive to temporal intrusion, and a planar mollusk with the black pearl of a new underworld accreting beneath its shell.
Tales of a "memaggot" that hides its own existence by eating thoughts as they putrefy into memories, and the ramblings of one amateur magician regarding a so-called "deep crow", are rightly dismissed as total bullshit.