Achelous

Achelous is a very large river god with a bull's head and a serpent's tail. He is the son of Oceanus and Tethys. One day he had a run-in with Heracles over the beautiful Deianira, and had one of his horns snapped off. This in turn was snapped up by Plutus, God of Wealth, who filled it with goodies and turned it into the Horn of Plenty.