Savage goliath has special/diffrent effects for his attacks, which is lovely.
Magma Kraken seems to have a couple of new sounds
Assuming this is the case, this means that there are two kinds of skins.
Savage = Premium
Magma = ???
Wendigo = Unknown, normal unlockable?
Elite = Monster only cosmetic, no new ability appearances or sounds.
Hopefully this means more Savage type skins.