Triumph of Neptune Standing on a Chariot Pulled by Two Sea Horses, mid-third century CE, mosaic, Sousse Archaeological Museum, Sousse
Poseidon (Greek) or Neptune (Roman) is the god of the sea. When the universe was divided into three, Zeus/Jupiter claimed the sky, Hades/Pluto took the underworld, and Poseidon/Neptune gained control of the sea. He married a nymph called Amphitrite.
Though not seen here, he often has long hair and is depicted with Amphitrite. His chariot is sometimes made of a conch shell.