Achelous was a deity in Greek mythology, the river god of the eponymous river, which is the largest in Greece. Therefore, Achelous was the most important of the river gods. Although some sources mention that he was the son of Gaea and Oceanus, it was generally believed that all river deities were descendants of Tethys and Oceanus. Achelous was depicted either as a gray-haired old man, or a bearded young man.
Achelous was defeated in contest by Heracles, while competing to win over the river nymph Deianeira, daughter of king Oeneus of Calydon. The rivalry between Achelous and Heracles was depicted on various Acarnanian coins, as a statue that was found in Olympia and on the throne of Amyclae. During the battle, Achelous transformed himself into a bull, but Heracles managed to tear one of his horns off, making him surrender. Achelous was forced to trade the horn of Amalthea to get it back. The horn of Amalthea was then given to the Naiads by Heracles, who transformed it into the cornucopia, a container full of fruit, vegetables and other produce.