Poseidon (Greek: Ποσειδῶν) was the god of the sea, and, as "Earth-Shaker," of earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon. Linear B tablets show that Poseidon was venerated at Pylos and Thebes in pre-Olympian Bronze Age Greece, but he was integrated into the Olympian gods as the brother of Zeus and Hades. Poseidon has many children. There is a Homeric hymn to Poseidon, who was the protector of many Hellenic cities, although he lost the contest for Athens to Athena.

[http://en.wikipedia.org/wiki/Neptune_%28mythology%29]

Neptune (Latin: Neptūnus) was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe, Heaven, Earth and the Netherworld. Depictions of Neptune in Roman mosaics, especially those of North Africa, are influenced by Hellenistic conventions.

Unlike the Greek Oceanus, titan of the world-ocean, Neptune was associated as well with fresh water. Georges Dumézil suggested that for Latins, who were not a seafaring people, the primary identification of Neptune was with freshwater springs. Like Poseidon, Neptune was worshipped by the Romans also as a god of horses, under the name Neptunus Equester, a patron of horse-racing.

[http://en.wikipedia.org/wiki/Poseidon]