From Fantasia Wiki
Achelous was the son of the sea Titans Oceanus and Tethys and the chief river god.
History
Circa 1260 BCE, Achelous traveled to Calydon in order to act as a suitor to Deianeira, daughter of King Oeneus and Queen Althea. Heracles was another of Deianeira's suitors, so Achelous challenged him. During the battle, Achelous transformed partially into a bull and charged at him, but Heracles managed to shatter his right horn.
Achelous held enmity towards Heracles for many centuries. In the early twenty-first century, he entered the service of Heracles' estranged cousin Eurystheus.