I will go with Firenze the Centaur. I think he might be the half-blood prince. The prophecy has to figure significantly in Book 6. Centaurs read prophesies from the stars and, in Book 1, all of them knew something about Harry Potter they weren't telling. Firenze trusts humans more than the other Centaurs. And face it, Firenze is half human and half horse so why not half prince?

OR half of a human and half of a horse, formerly known as Prince.