> Gargoyle has wings but why they cannot ride on horse/pets?
I think that was answered in Ultima 6. The gargoyle companion refuses to sit on food.
> Gargoyle has throwing why human cannot?
That's peculiar to Ultima Online. In Ultima 6, a human companion could pick up a boomerang and use it.
Ultima 7 had several thrown weapons that would usually return to the hand: magic axes, juggernaut hammer, hawk. I think there was a magic boomerang too. The "usually" was problematic. The weapon could get caught in trees and be lost. Or it could hit another companion and do damage.