Roman name Neptune. See The Olympians for another picture. Poseidon was the god of the sea, earthquakes and horses. Although he was officially one of the supreme gods of Mount Olympus, he spent most of his time in his watery domain. Poseidon was brother to Zeus and Hades. These three gods divided up creation. Zeus became ruler of the sky, Hades got dominion of the Underworld and Poseidon was given all water, both fresh and salt.