Achelous is the son of the Titans Oceanus and Tethys, gods of the ocean. On earth, he ruled the river Achelous which shared his name. the river flowed southward from Thessaly and formed the boundary between Acarnania and Aetolia as it flowed to the Ionian Sea in the land that would be modern Greece.
When five native earth-goddesses known as nymphs refused to honor him, Achelous swept them away to the Echinadian Islands. He later fell in love with Perimela, daughter of the mortal king Aeolus, who flung her off a cliff, but Achelous persuaded the sea-god Neptune to rescue her for him. Perimela bore him three daughters, Callirrhoe, wife of the seer Alcmaeon; Peirene, protector of a spring at Corinth and Castalia, protector of another spring at Delphi. After Perimela died, Achelous fell in love with the Muse Melpomene, daughter of Zeus, King of the Olympian Gods, and the Titaness, Mnemosyne. They were the parents of the Sirens who actually began to excel in singing over their mother and their aunts. The Sirens were then exiled to the island Anthemoessa near Ancient Italy far from the ears of their vain aunts.
Around the Thirteenth Century BC, Achelous fell in love with Deianeira, a Caledonian princess, but she was already in love Hercules, the son of Zeus. Believing that he could make a better husband for Deianeira than the frequently carousing son of Zeus, Achelous met Hercules in unarmed combat to fight for her hand. He tried to tire out the hero by shapeshifting through several forms to tire him out, but Hercules held fast to his forms and when Achelous became a great bull, Hercules wrestled him down and ripped one of his horns off effectively and painfully ending the fight. Hercules, however, filled the horn with food and left it as an offering to the gods and it became known as the Cornucopia, or Horn of Plenty.
Under unrevealed circumstances, Achelous became a ward of the former king of Mycenae, Eurystheus, since restored to life by the goddess, Hera.
After several years of distant enmity with Hercules, Achelous was sent to meet with Hercules at the Extra Points Bar where he confronted him in the men's room as Hercules used the facilities. Getting in the first punch, he knocked Hercules through the door then reminded him of who he was. He then dragged Hercules out to the back of a limousine where Eurystheus was waiting.
Dragging Hercules into Eurystheus's modern corporate offices, Achelous and Eurystheus rehashed past events with Hercules and his film crew over several years of events. While in human form for most of the time, Achelous took offense and took on his bull form when Robert the Camera Man called him the world's first stalker. Threatening the young man, Hercules knocked Achelous cold. Still wanting to get even with Hercules, Achelous had his temper stalled by Eurystheus just as Hercules' friend James rushed out apologizing for despoiling the bathroom facilities.
Achelous and Eurystheus provide the TV commentary as Hercules and his film crew go around the world in a modern day version of the twelve labors with modern equivalents to the old creatures and objects of myth. As they realized that Hercules was tearing through all their new obstacles, Hera appeared to them with a set of four labors designed against Hercules.
Achelous stood by as Eurystheus gave Hercules his last modern labor: to enter the underworld and retrieve a flower from the hair of his first wife, Megaera.
Achelous overheard through Hera that Hercules had succeeded in the last of his modern labors. upon hearing Hercules charging to confront them, Hera made a hasty departure. Eurystheus then flicked off the lights of his office so that Achelous could deal with his unfinished business. A flash of Robert's camera gave Hercules a chance to deal with Achelous and Achelous ended up rendered on the tip of Eurystheus's sword with the former king impaled on his own bull horns.