Poseidon on a red-figure vase shown wielding his trident.

Poseidon is striking Polybotes, one of the giants. There are different stories about the origins of the giants, but in one, Gaia (seen in this picture) produced the giants for revenge after the Olympians defeated the Titans in the Titanomachy. Polybotes is described as the son of Gaia whom Poseidon killed or at least placed beneath the island of Cos.