I was thinking back from my old D&D days and came up with this list of stuff you might find underground and a brief note about their use/abilities:
Future creatures
Lizardman Farmer?
Mind Flayer control others
Dark Elf High Magic Use
Succubus charm
Deep Dwarf miner. Perhaps they have the ability to 'diffuse' cave ins by shoring up the stone?
Crawling Chaos Summon Tenticle
Dragonkin Breath weapon
Spider Random enemy
Ooze Random enemy. Absorbs resources/items
Living Shadow Needs a light source. Drains will (morale?)
Skeleton Graveyard room?
Zombie Graveyard room?
Vampire Turns humans to vampires
Demon Dangerous Unpredictable
Devil Organized Intelligent
Basilisk Petrify
Sloth Poison
Hell Hound Fire Damage, Phase through walls
Troll Regenerates
Werewolf Turns humans to werewolves
Wererat Steals. Can swim
Beholder Flying eyeball of death and doom... Should by enemy only
Gargoyle Immune to damage in stone form
Future creatures
Lizardman Farmer?
Mind Flayer control others
Dark Elf High Magic Use
Succubus charm
Deep Dwarf miner. Perhaps they have the ability to 'diffuse' cave ins by shoring up the stone?
Crawling Chaos Summon Tenticle
Dragonkin Breath weapon
Spider Random enemy
Ooze Random enemy. Absorbs resources/items
Living Shadow Needs a light source. Drains will (morale?)
Skeleton Graveyard room?
Zombie Graveyard room?
Vampire Turns humans to vampires
Demon Dangerous Unpredictable
Devil Organized Intelligent
Basilisk Petrify
Sloth Poison
Hell Hound Fire Damage, Phase through walls
Troll Regenerates
Werewolf Turns humans to werewolves
Wererat Steals. Can swim
Beholder Flying eyeball of death and doom... Should by enemy only
Gargoyle Immune to damage in stone form