Fox-morph TF grants you 'Icy Veins' after enough uses, I think. It's quite useful if you're exploring Uveto and dont' want to switch clothing/equipment, albet with the addition of better equipment with high cold-protection stats, that usefulness has lessened.
Avian-morph TF gives you 'Hollow bones', and from I've heard, the only think that it does is reducing your body weight and the amount of weight that you can carry. The same transformative also gives you wings, that are quite handy in a couple of scenarios (Dove Balm gives them too, if you want to avoid the more bird-like changes on your body)
Cat-morph TF gives you 'Flexibility', that allows you to use the auto-fellatio masturbation.
Red myr TF gives you venom, as far as I know, that gives you a couple of special scenes with a few NPC.
Horse TF gives you a equine mane. It's purely a cosmetic thing. I think that the same TF gave you a negative perk, about being slow (mentally speaking) but I'm not sure about that.
Huskar TF gives you a mane too, I'm unsure if that's purely cosmetical or brings the same cold-resistance benefit that 'Icy Veins'.
Goblin TF gives you an increase in hip and bust size, and it may give you an increased fertility ('Fecund Figure, I think it's called').