Poseidon was the god of the Mediterranean sea. He was a son of Cronos and Rhea and so accordingly a brother of Zeus, Hades, Hera, Hestia and Demeter, and it was determined by lot that he should rule over the sea.

Although he generally dwelt in the sea, still he also appears in Olympus in the assembly of the gods.

Poseidon of Artemision statue

Bust of the Poseidon of Artemision

Poseidon in conjunction with Apollo is said to have built the walls of Troy for Laomedon. When the gods finished building the walls, Laomedon refused to give them the reward which had been stipulated, and even dismissed them with threats. Poseidon sent a marine monster, which was on the point of devouring Laomedon's daughter, when it was killed by Heracles.

For this reason Poseidon like Hera bore an implacable hatred against the Trojans and took an active part in the war against Troy, in which he sided with the Greeks, sometimes witnessing the contest as a spectator from the heights of Thrace, and sometimes interfering in person, assuming the appearance of a mortal hero and encouraging the Greeks.

When Zeus permitted the gods to assist whichever party they pleased, Poseidon joining the Greeks, took part in the war, and caused the earth to tremble; he was opposed by Apollo.

In the Odyssey, Poseidon appears hostile to Odysseus, whom he prevents from returning home in consequence of his having blinded the Cyclop Polyphemus, a son of Poseidon by the nymph Thoosa.

He was further regarded as the creator of the horse as follows:-- when Poseidon and Athena disputed as to which of them should give the name to the capital of Attica, the gods decided, that it should receive its name from him who should bestow upon man the most useful gift. Poseidon their created the horse, and Athena called forth the olive tree, for which the honour was conferred upon her.

Black figured Oinochoe
Poseidon amongst the gods watching Hercules' introduction to Olympus.
6th century B.C. National Archaeological Museum, Athens.

Poseidon, part of our
12 Olympian gods collection

The symbol of Poseidon's power was the trident, or a spear with three points, with which he used to shatter rocks, to call forth or subdue storms, to shake the earth, and the like. Poseidon was married to Amphitrite, by whom he had three children, Triton, Rhode, and Benthesicyme; but he had besides a vast number of children by other divinities and mortal women.

In works of art, Poseidon may be easily recognised by his attributes, the dolphin, the horse, or the trident and he was frequently represented in groups along with Amphitrite, Tritons, Nereids, dolphins, the Dioscuri, Palaemon, Pegasus, Bellerophontes, Thalassa, Ino, and Galene.

Athenian Hoplites chess set
featuring Poseidon as King

Source: Dictionary of Greek and Roman Biography and Mythology