All Monsters (9)

• Aboleth - Ancient race of fish-like amphibians with three eyes and four tentacles
  Field Guide
• Cheiropteran - Bat-people of the deep under-dark with bugbear-like bodies and misshapen bat heads
  Field Guide
• Dark Creeper - Short, spindly humanoids with pale skin who wrap themselves in loose, dark clothing
  OSRIC
• Deep Spore - Knobby leathery globes 2-5 feet in diameter with large eye spots that float in underground realms
  Field Guide
• Dragon, Cave - Solitary, territorial dragons dwelling in the deepest caverns with exceptional echolocation abilities
  Field Guide
• Golem, Lead - Ancient construct sacred to the bat-god Camazotz, man-shaped with bat head and wings
  Field Guide
• Underdark Monsters in Basic Fantasy Field Guide Omnibus
  
• Underdark Monsters in OSRIC
  
• Worm Man - Humanoid worms that live deep underground, about 7 feet long with two arms and excellent peripheral vision
  Field Guide