According to the "Iliad" of Homer, when the three sons of Cronus and Rhea divided up the world by lot, Zeus became the god of the sky, Hades became the god of the underworld, and Poseidon became the god of the sea. In Greek mythology, Poseidon was not only the god of the sea and water, but also of earthquakes and horses.
In Greek mythological tales, Cronus, fearing his offspring, ate them as soon as they were born. When Zeus was born, Rhea substituted a rock wrapped in swaddling cloth that Cronus ate and then fled with Zeus to Crete. Later, Zeus confronted his father, castrated him and forced him to regurgitate his other offspring, among them Poseidon. In other versions of the tale, Poseidon escaped being eaten, as Zeus did. Though ostensibly Zeus' equal, Poseidon deferred to Zeus as king.
Poseidon is usually pictured as a bearded, mature man. According to About.com, his symbol is the three-pronged trident, or fish spear. According to Greek mythology, when the Titans made the thunderbolt for Zeus, they made the trident for Poseidon. He was thought to be responsible for drownings, shipwrecks and earthquakes. Though he had a wife named Amphitrite, he mated with many other gods and mortals. The Cyclops Odysseus encountered was one of his children. When he mated with Medusa, the result was Pegasus, the winged horse. Poseidon had many similar attributes to the Roman god of the sea, Neptune.