Poseidon is the Greek god of the ocean. He is one of the twelve ruling Olympian Gods. His symbolic creature is the horse and his symbol is the trident.
He is brother to Zeus, Hades, Hera, Demeter and Hestia. His father is Kronos and his mother is Rhea . He is uncle to Hephaestus, Ares and Persephone. His children are Triton, Pegasus, Theseus and Bellerophon. He's in/was in relationships with Amphtitrite, Medusa, Aethra and Eurymede.
At the creation of the city Athens (which at that point was not named) Poseidon and Athena gave gifts to the city so they would name the city after one of them. Poseidon struck the ground with his trident and a spring shot forth. The people were amazed, but when they tasted the water it was salty, like the oceans he rules. Then Athena knelt and buried something in the dirt. The thing she planted was a seed for an olive tree, which is her symbolic tree, which was chosen as the better gift because it gave the city food, oil and wood. They decided that the city would be named Athens, after Athena. Poseidon and Athena disliked each other strongly from that day forth.
Poseidon's Roman alter ego is Neptune.
Poseidon is known as the "Father of Horses", as he created horses from the sea. He can also communicate with and command horses, as well as other creatures of the sea. In mythology he is seen as one of the most powerful but also aggressive of the gods, and often caused earthquakes in his rages.
Poseidon has been seen to create giant tidal waves by flying in from the sky at high speeds and slamming into the sea. The resulting shockwave from his impact creates the vast wave that can destroy an entire fleet of war ships. He can fire magic energy from his trident to transmutate matter (transforming humans into water and sea monsters into harmless swarms of fish).