Achelous

Achelous is a bull-man river god, the god of his own river named after himself.

History
Achelous was prepared to marry his soon-to-be wife Deianira, until Hercules saw her, and challenged Achelous for Deianira. During this fight, Hercules had cut off Achelous horn and used this as a way to humiliate him and Deianira. Deianira had picked Hercules over Achelous, leaving Achelous bitter and jealous of Hercules.

The Mark of Athena
Jason Grace and Piper McLean are sent on a quest by Hercules to go take Achelous second horn to humiliate him even more. When Jason and Piper get to his river, he explains to them why Hercules did not deserve his second horn. Despite how bad they felt for him, they fought with him for the horn, which Achelous begins to try to drown them with his river. Piper cuts off his second horn and her Jason escape the river, leaving Achelous.