Poseidon is the powerful Greek god of the sea, earthquakes, storms, and horses, known for his trident and sometimes violent temper, ruling the oceans alongside his wife Amphitrite, and is the brother of Zeus and Hades, with the Roman equivalent being Neptune. He's a major Olympian deity, responsible for both safe sea travel and destructive tempests, and is depicted in myths as creating springs and fathering creatures like Pegasus and the Cyclops Polyphemus.