Poseidon was the Greek god of the sea, and one of the three most powerful gods in their mythology. He was the brother of Zeus and Hades. When the three brothers divided up the world after defeating the Titans, Poseidon got the oceans, rivers, and every drop of water in between.
His weapon was a trident, which is basically a fishing spear with three sharp prongs. According to the myths, he could stir up giant waves, sink ships, or create freshwater springs by stabbing it into the ground. Sailors prayed to him for safe journeys and made offerings before long voyages.
The ancient Greeks also believed Poseidon invented horses, which is why he was sometimes shown riding a chariot pulled by sea-horses with fish tails. His Roman version was called Neptune, which is also the name of the eighth planet in our solar system.