Asterion, the Minotaur

From Greek mythology.

A monstrous bull-man, son of Pasifae and a sacred bull. A name of "Minotaur" wasn't used in Crete - there he was called "Asterion", like foster-father of Minos.

I think it could be a real creature - just a mutated Minos' son who wasn't killed because of his aristocratic origin and who was imprisoned by his eccentric father in a cage-like contruction which we call " the Labyrinth". Because of his deformated head ancient people could associate it with a bull, which started this all of rumors about the alleged Pasifae's zoophilia to slander her and all of dynasty.