Ah, Poseidon. According to mythology, he was the Greek god of the sea who also happened to be associated with earthquakes.
Here are some of the main symbols of the god Poseidon:
Bull - In ancient Greece, bulls were sacrificed to Poseidon (as well as the other gods). One legendary example is the the bull of Minos. In case my teasing reference to that story isn’t ringing any bells just yet, here is another hint - the Minotaur.
Fish - As the god of the sea, it stands to reason that fish and other aquatic creatures would be associated with Poseidon.
Horses - Poseidon was considered to be a god of horses. The title Hippios (”Lord of Horses”) reflects this role, and there are several myths in which the god appeared in the guise of a horse.
One of my favorite depictions of the god and his equine symbol is the painting Neptune’s Horses by Nineteenth century artist Walter Crane.
Trident - This distinctive type of three pronged spear was used by tuna fishermen, so it’s no surprise to see the god of the sea wielding this weapon as well.