Achelous was a suitor for Deianeira, daughter of Oeneus king of Calydon, but was defeated by Heracles, who wed her himself.

When he battled Heracles over the river nymph Deianeira, Achelous turned himself into a bull. Heracles tore off one of his horns and forced the god to surrender. Achelous had to trade the goat horn of Amalthea to get it back. Heracles gave it to the Naiads, who transformed it into the cornucopia.

Achelous was sometimes the father of the Sirens by Terpsichore, or in a later version, they are from the blood he shed where Heracles broke off his horn. In another mythic context, the Achelous was said to be formed by the tears of Niobe, who fled to Mount Sipylon after the deaths of her husband and children.

In Hellenistic and Roman contexts, the river god was often reduced to a mask and used decoratively as an emblem of water.