Achelous is a fiend in Final Fantasy X. It is encountered in the water of the Mt. Gagazet caves. Being a Water-based monster, it is vulnerable to Lightning-based attacks. Once it has been physically attacked, it will begin to charge, and on its next turn will unleash Sonic Wave on the party. If Achelous is put to Sleep while glowing, it will have to charge again.
In Greek mythology, Achelous was the deity of the largest river in Greece.