Well, for starters, giant versions of all the existing cave vermin. Giant cave lobsters, giant cap hoppers, giant purring maggots (which can be domesticated and milked, of course!)
Some more slimy blobs would be nice. Maybe borrow from the 'blobs, slimes, and jellies' mod.
Regular critters with 'naked mole' or 'blind cave' variations.
Things with tentacles. Lots and lots of things with tentacles.
Dinosaurs and other prehistoric creatures. 'Cause fossils are underground. Or something.
Tarrasque would make an excellent megabeast.
I did some research to find some real-life cave-dwelling creatures. Most are tiny and kind of boring, but we need some creature variation to add color to the world. Metaphorically speaking, as most of them are colorless. Giant variants could be fun.
Pseudoscorpion! A cave-dwelling spider-like creature that injects paralytic venom through its scorpion-like pincers, spins webs from its mouth, and spits corrosive fluid to pre-digest its prey. We've got to have this, no question. And a giant version of course.
Phantom cave snail. Nothing too special about it, but you've got to like the name.
Various aquatic cave snails.
Cave shrimp.
Cave spide...nevermind.
Blind cave millipede (herbivore).
Cave centipede (carnivore).
Cave moth.
Cave grasshopper (predatory!)
Leptodirus hochenwartii, or simply 'cave beetle' (a scavenger, so [VERMIN_ROTTER] tag).
Anophthalmus hitleri, or 'blind cave beetle'. A predator. Yes, the scientific name is intentional.
Springtail, an insect with a long tail that it uses to jump.
Waterfall climbing cave fish. A rare freshwater fish that can grapple terrain with its fins and climb walls.
Pupfish. Freshwater or brackishwater fish.
Blind cave catfish. Freshwater fish with tiny, probably functionless eyes, can breathe air.
Blind cave tetra. Freshwater, eyeless.
Blind cave loach. Freshwater.
Cave knifefish. Freshwater or brackishwater. Generates a weak electric field, but only for navigation, not a weapon. No knives either. What a shame.
Cave eel. (not technically eels, but meh)
(all of these fish are small)
Oh, and four words:
Plump. Helmet. Man.
Civilization.