These beasts live within dark caverns and caves. Each one has random traits and abilities; roll once in each category. You could also use this to generate random monsters for Underark creatures as well, but you would need to replace some of the results for the implied setting there. Replacing the 'dangling goblin corpse' with a drow corpse, for example. Inspired by Dwarf Fortress Forgotten Beasts and their potentially insane generations.
The Body – 1d10
 Humanoid (3 HD, 12 AC) Has a pot belly and deathly thin everywhere else. It's head leers down over you. It's appearance belies a malevolent intelligence- it can use weapons, but doesn't need to.
 Beast Head (5 HD, 9 AC, Bite deals 1d10) Massive head, looks like an ape mixed with a bear. Moves on a billion slithering tentacles, relies on scooping prey into its mouth.
 Bat (4 HD, 14 AC, Flight) Thin, huge wings that fly in silence.
 Worm (6 HD, 8 AC) It can slink with frightening speed.
 Fish (3 HD, 16 AC, Hovering) Appears as a massive scaly fish that swims through air.
 Lizard (4 HD, 14 AC, Tongue-Lash) Six legged gecko. Has a long tongue attack that draws people in to its mouth on a failed save.
 Turtle (3 HD, 17 AC) Like an armored tortoise with pitch black spikes growing out its back. Has a dangling goblin corpse impaled on one of the spikes, smells of death.
 Herbivore (5 HD, 14 AC, no bite attack, gains two hoof attacks at 1d6) Knobbly legged wildebeest. It doesn't belong here, poorly suited to this environment.
 Blob (8 HD, 6 AC, Moves over/through all terrain, very slow) Flows as honey, with murderous intent. You can escape it easily, but pursues relentlessly.
 Insect (4 HD, 12 AC, cannot be flanked or surprised) Six spindly insect legs over a short, fat, grub of a body. No head, only holes on each end. Both holes do each others job; eat and excrete.
Eyes – 1d8
 Red (+2 to hit) Murderous, filled with a hate that is beyond instinct.
 Unblinking (+1 HD) Primordial and uncaring. Appraises you without thought.
 Yellow (+2 to saves) Pure glowing color, gives it the appearance of being an artificial thing.
 Spikes (+1 damage to all attacks) Instead of eyes, it has dark metal spikes jutting out.
 Spider (Can fire Web once per combat) Many small, black eyes on its head and face.
 Painted (Gains +1 AC, cannot be blinded except Holy Water) It's eyes are drawn on, and its vision can only be stopped by scrubbing them away. Regular water has no effect.
 Holes (Can cast a single 1st or 2nd level spell, once per combat) Hollow eyes, it's enchanted by something dark and sinister.
 Cyclopean (Fire Eye Laser once per combat, save vs beams, deals 1d6) Sweeping beams emit from this single large eye.
Material/Skin – 1d8
 Hot (Aura of Heat, save or sweat each round in melee. Save again each round or take 1d4 nonlethal fire damage, beast takes ½ damage from fire spells) Appears like cracked magma, water steams on touch.
 Metal (+2 AC) The metallic skin of the beast grinds as metal. If skinned, this could be forged into a suit of unnaturally flexible metal armor; AC as leather+1
 Scaly (Immune to Slows, Sleeps, and Stuns) Its looks like something lost to time.
 Matted Fur (Takes ½ damage from cold, go into rage gaining +2 to hit and damage but -2 AC when reaching half health) The beast is mangy and hungry like a wild wolf.
 Wrinkled (Takes ½ damage from daggers and spears, cannot be poisoned or hurt at all by darts, ninja stars, etc.) It's body is wrinkled and clammy, smells of dust between its folds.
 Porcelain (Ignores Bronze Armor & Weapons, +2 to hit with all attacks) Alabaster white, a crafted being, looks like it belongs among the rubble and columns of ancient cities.
 Cloud (Immune to nonmagical weapons and attacks) Barely present, it's smoking body is a suggestion of a form. White spectral eels dance in its gut.
 Infested (Choking Spores fill the chamber- save or cannot speak/cast spells and take 1d4 damage, Treat as Undead, but +2 HD for Turning Attempts) The creature looks half dead, rotting flesh with mushrooms growing from every spot. Spores fall with each shake. Anything this creature kills starts to sprout in 1d4 rounds, which also release spores when disturbed.
Bite Attack – 1d4
 Molars (1d6+1 crushing damage) Giant molars that grind and squeeze.
 Canines (1d8, -2 to saves to escape) Sharp, pointed teeth, keeps prey in place.
 Lamprey (1d6, heals for half damage done on bites) Blood spilling, horrible spiral wounds.
 Fangs (1d6, save vs poison or take 1d6 damage per round each round) Like a snake, huge fangs jut out when teeth are bared. Dripping with sizzling venom.
Powers – 1d10
 Claws (Gains a claw attack at 1d4) Two shrunken arms on chest, rip anything pressed up close.
 Javelins (On first hit taken, all nearby must save or take 1d6 damage from ejected spears) It's body is covered in rusted weapons from an ancient attempt to slay the beast. Clearly, they failed.
 Minions (After taking 3 hits, gives birth to 2d6 tiny 1 HD monsters. The monsters have one attack at 1d4 and are nearly blind and deaf) The creature is pregnant. After taking significant damage, it gives birth to a squealing litter; the undeveloped fetal spawn as monstrous as their parent and born weaned; ready to taste flesh.
 Gaze of Pain (Each round, one target the beast looks at must save or take 1d4 damage. They feel horrible pain and lose concentration on spells) This beast's eyes emit malice. Anyone who meets them will feel a horrible pain; inflicted from within the beast's black soul.
 Shifting Walls (If the beast is killed, the area the party is in subtly warps without their knowledge. They are shifted to another part of the dungeon, potentially on another floor, randomly.) The beast's last laugh; a gift from the minotaur.
 Metamorphosis (Creature can stop to transform its Body, Material, and/or Bite attack. It keeps its original HD and hit points but changes AC and other stats according. Transformation takes one round that cannot be used to attack or use abilities) Its flesh is as clay, shifting into new forms to dominate in any ecosystem, or to fight any foe.
 Occult (Creature can cast either one 3rd level spell, OR two 1st or 2nd level spells each day. The spells are randomly determined, but should be weighted towards curses and offensive magic) The being possesses a magical presence. Large, dusty gray moths land on its surface when it is still and swarm around it when it moves.
 Swallow (Can swallow up to one medium sized creature at a time, deals 1d4 acid damage per round internally to that person, can only cut yourself out with an attack roll of 19 or higher with a dagger or claws inside) The creature has a belly or pouch and a large mouth. Voracious.
 Anomalous (Takes no damage from the first three attacks or spells that hit it. If it is hit by an anti magic spell; roll again on power table and it warps to that) The being's skin and flesh sizzles and bubbles, the smell of flowers fill your mind with memories you never had.
 Ash Field (Extinguishes 1 torch within melee range per round for Ash, takes ½ damage from Fire spells and converts spell level to Ash, If material is “Hot”, gains 1 Ash per round. (Ash is used by the creature to create a swirling shield around it of +2 per Ash used per round OR it can roar/screech/rumble and throw up ash that fills a room which causes temporary blindness & inability to cast spells.) The creature is warm, its skin flakes with the sound of crackling fire.