the god of the sea
One of the three killers of Kronos
As the evil Kronos kept eating the children Poseidon was one of the three that made it out of his grasp and killed him. Making him the ruler of the sea. He could have been the ruler of the underworld, but he did not like the underworld. He could have been the king of all gods, but he didn't want that much responsibility.
The all powerful trident
As Poseidon had his job of the god of the sea, he had to protect the sea animals. His trident was very helpful and it has three points on the end of it.