Poseidon is the brother of Zeus. After they had overthrown their Father Cronus he drew lots with Zeus and Hades, another brother, for shares of the world. His prize was to become lord of the sea. He was widely worshiped by seamen. He married Amphitrite, the granddaughter of the Titan Oceanus.
At one point he desired Demeter. To put him off Demeter asked him to make the most beautiful animal that the world had ever seen. So to impress her Poseidon created the first horse. In some accounts his first attempts were unsuccessful and created a variety of other animals in his quest. By the time the horse was created his passion for Demeter had cooled.
His weapon is a trident, which can shake the earth, and shatter any object. He is second only to Zeus in power amongst the gods. He has a difficult quarrelsome personality. He was greedy. He had a series of disputes with other gods when he tried to take over their cities.