Poseidon, in ancient Greek religion, god of the sea (and of water generally), earthquakes, and horses. He is distinguished from Pontus, the personification of the sea and the oldest Greek divinity of the waters.

Neptune, Latin Neptunus, in Roman religion, originally the god of fresh water; by 399 bce he was identified with the Greek Poseidon and thus became a deity of the sea. His female counterpart, Salacia, was perhaps originally a goddess of leaping springwater, subsequently equated with the Greek Amphitrite.

Although Greek Gods are arguably better known, Greek and Roman mythology often have the same Gods with different names because many Roman Gods are borrowed from Greek mythology, often with different traits. For example, Cupid is the Roman god of love and Eros is the Greek god of love.

Greek Gods and their Roman Counterparts

Greek God (English name)	Roman Counterpart	Domain
Aphrodite	Venus	Goddess of Love
Apollo	Phoebus Apollo	God of the Sun
Ares	Mars	God of war
Artemis	Diana	Virgin goddess of the hunt, wilderness, wild animals, childbirth and plague. In later times she became associated with the moon.
Athena	Minerva	Goddess of wisdom
Demeter	Ceres	Goddess of grain/crops
Dionysus	Bacchus	God of wine
Eros	Cupid	God of love
Hades	Pluto	God of underworld
Hecate	Trivia	Goddess of witchcraft, crossroads, and justice
Helios	Sol	The sun God
Hephaestus	Vulcan	God of fire, and the forge
Hera	Juno	Queen of the Gods
Hermes	Mercury	Messenger of the Gods
Nike	Victoria	Goddess of victory
Pan	Faunus	God of woods and pastures
Poseidon	Neptune	God of the sea
Zeus	Jupiter	King of Gods

The Romans more commonly used the name of their own God but it was not uncommon to see in literature and art the Greek form of the name.

I wanted one more centrepiece for the Roman seaside village of Stabiae. I was hoping to find a small Roman statue that I could use but ended up with this Alabaster one of Poseidon for a few dollars in a car boot sale.

Poseidon is slightly larger than I would of liked but still looks OK.

After all he is a GOD!