Outside the Tower, animals are broadly Earth-like. Inside, unique organisms exist; their origin is unknown.
Amended D-293 The world outside is fully mundane: no trolls, elves, dragons or magic, not even as real rarities. Everything strange in the world comes from the Tower.
Tower fauna should feel biological but unfamiliar. Not every creature must be radically original. The ecosystem may contain readable fauna, unfamiliar fauna and rare incomprehensible fauna.
Useful design tools: unfamiliar locomotion, anatomical ambiguity, alien materiality, translucency, radial/distributed anatomy, unusual sensory systems.
A standard creature usually needs one strong alien biological idea, occasionally two. Do not pile every strange feature onto every monster.
Most Tower creatures are larger than humans, but the Tower remains vastly larger than them. Creature bodies should naturally provide materials usable by the creature-derived equipment system.
Amended D-302 Most Tower creatures are inedible to humans: their bodies are material, not meat. That is part of why nobody can live in the Tower (§31).
Amended D-329 They live off their own ecosystem: each other, moss, and things humans cannot digest. An enemy's bag in the game is a representation of what the creature holds, whether in its gut, caught in its hide or kept in its hoard, never a literal bag; nothing should be drawn carrying one. Human gear and food found on a creature were taken from the dead, out of habit or nesting instinct. How the ecosystem sustains itself stays a mystery (§2).
Some species may opportunistically use Tower fragments for temperature regulation, buoyant nests, armor, light or crude tools, but this is not universal.