Metamorphoses Book 9: Achelous and Hercules
Achelous told Theseus and his men the story of his battle with Hercules. Achelous and Hercules were fighting over Deianira. Achelous had been in human form, but he couldn't outmatch Hercules, so he changed to a snake. From the snake, he transformed himself to a bull, and Hercules pinned him to the ground and broke off one of his horns. The broken horn was made the cornucopia by his water nymphs.
Theseus and his men left Achelous' cave the next morning and Achelous turned back into water.