Poseidon was the Greek god of the sea and rivers, creator of storms and floods, and the bringer of earthquakes and destruction. He was perhaps the most disruptive of all the ancient gods but he was not always a negative force. He was a protector to mariners and, as a tamer of horses, the patron of that animal and horse breeding. To the Romans, he was known as Neptune.
Cults to Poseidon date as far back as the late Bronze Age and the Mycenaean civilization (at its peak from the 15th to 12th century BCE), as attested by Linear B inscriptions found at Pylos in the Peloponnese and Knossos on Crete. Indeed, the god seems to have been one of the most important Mycenaean deities, perhaps no surprise given the culture's obvious seafaring skills. It may be that Poseidon was a mix of an indigenous but pre-Greek god with Potis, an Indo-European deity. Pylos, we know, had Poseidon as its main god with a priestess as the head of his cult.
In later Greek mythology, Poseidon was the son of Kronos and Rhea, and brother of Zeus and Hades. He was a key figure in the battles for control of the universe between the Titans, the Giants, and the Olympians. On their victory, the three brothers drew lots to decide which domain they would reign over, and Poseidon gained the seas. The god dwelled in magnificent golden mansions beneath the sea, beautifully adorned with coral and sea flowers. Traditionally, this undersea palace, which included the god's stables of fine white horses, was located near Aegae in Euboea. Seemingly not content with the seas alone, Poseidon often interfered in the plans of Zeus, and once even attempted to overthrow his brother with the aid of Hera and Athena. It was as punishment for this treachery that Poseidon was made to build the magnificent walls of Troy.