This doesn't answer the whole question, but here's a start...

Abominable
All Hallow's
Candy Corn
Charnel
Ectoplasm
Eldritch
Ghoulish/Vampyre
Jack O' Lantern
Pumpkin
Roynish
Squamous

Trick or Treat - turns into a different species when tamed

Fear Evolved Color Sets
https://steamcommunity.com/sharedfiles/filedetails/?id=2557505983

Ghoulish has a cool added design, The others seem more color based. But how is Eldritch, Roynish, and Squamous just colors? I wish there were simple explanations given for them.

(I don't know if they're just colors; that is only all the information I personally have at this moment.)

They're all just colours, yes. Trick-or-treat is the only one with an extra special surprise.

How can I disable Tek Variants for the Trick or Treat? Anyone know?