Achelous

Achelous is a bull-man and is the god of the river "Achelous", which is named after him. The river is situated on the island where the Pillars of Hercules are located.

History
Achelous was going to marry Deianira, the Princess of Clydon, until Hercules saw her, and challenged Achelous for Deianira. During this fight, Hercules cut off Achelous' horn and used this as a way to humiliate him; the horn later turned into a cornucopia(or the horn of plenty). Deianira 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 retrieve Achelous' other horn to humiliate him even further, since Hercules had cut off his first one. When Jason and Piper get to his river, Achelous is singing. Piper thinks that the singing has the same effects as charmspeaking. Achelous assumes that the demigods are there to kill him at the request of Hercules. He explains to them why Hercules did not deserve his horn and tells them the story of Deianira. Although both Piper and Jason feel bad for Achelous, who seems to be friendly, the god tries to stop them by drowning them. Whilst Jason is pulled under, Piper tries to bargain with Achelous, but the god believes that she wants to be his bride, to compensate for Deianira. When Achelous picks her up, she threatens him with her with her knife to let Jason go. Piper then cuts off the god's horn and leaps for the shore, using Jason's power over the winds to carry her safely to the bank. Achelous wails as he sinks back into the river to mourn his horn.

Hercules never gets Achelous' horn because Piper forces him to give them permission for their ship to be able to leave. Piper uses the horn as a cornucopia at Hercules, and food comes pouring out of it, burying the god. The Argo II flees from Hercules' island, leaving behind a mourning Achelous and an angry Hercules.

Appearance
Achelous is a bull with a human face, with short curly black hair, a ringlet beard in the style of Ancient Greece. His skin is blue like his river. His eyes seem deep and mournful behind bifocal glasses and he has a pouting mouth. He has a single, curved, black-and-white bull's horn that sprouts from the left side of his head.