I'll start from Tier 1-2-3-4-5 and go Fire,Battle,Magic.
Fyora: Raptor-like creature with ability to spray fire.
Thahd: I don't know, but this always reminds me of men made out of clay, (Prometheus)
Artila: A worm.
Roamer: If you look at the Geneforge 5 picture with Rawal, they are shown to look like dogs.
Clawbug: A scorpion.
Vlish: On a small scale, they look like a body part (disembodied heart, with veins and such) or food.
Drayk: A crawling variation of lizard/dragon.
Battle Alpha: In the older games, they look like gigantic Neanderthals, in G4/G5, they look more hairy, however those markings on them persist, perhaps some sort of tribal human.
Glaahk: Ok.. this looks like a Tyrannosaurus without hands, a bug head and it's tail on backwards to me.
Krysarkk: Like another dinosaur.
Rotgroth: It looks like someone tall who needs immediate medical attention!
Wingbolt: I couldn't quite tell what looked like a Wingbolt until I saw a hawk flying with a squirrel in its clutch, to make a winged creature, however with a long, curved tail.
Drakon: Humanoid dragon.
War Thrall: Sort of like.. a troll from mythology with a pouch.
Gazer: No idea.
This is one of my first posts.