The Posseidon, Oceanos and Thetys mosaic.

From the Enc. Britt.: In Hesiod's Theogony, Oceanus was the son of Uranus (Heaven) and Gaea (Earth), the husband of the Titan Tethys, and father of 3,000 stream spirits and 3,000 ocean nymphs. In Homer's works he was the origin of the gods. As a common noun the word received almost the modern sense of ocean.

And on Poseidon: in Greek religion, god of the sea and of water generally; he is to be distinguished from Pontus, the personification of the sea and the oldest Greek divinity of the waters. The name Poseidon means either "husband of earth" or "lord of the earth." Traditionally he was a son of Cronus, an ancient chief god, and Rhea, a fertility goddess, and was brother of Zeus, the chief god, and Hades, god of the underworld. When the three brothers deposed their father, the kingdom of the sea fell by lot to Poseidon. His weapon was the trident, but it may originally have been a long-handled fish spear.

Poseidon was also the god of earthquakes, and many of his oldest places of worship in Greece were inland. He was, in addition, closely associated with horses.