The Greeks knew Poseidon best as lord of the sea, but his oldest and most persistent title is Enosichthon / Ennosigaios, the Earth-shaker. He was the god of earthquakes, and the Greek world, sitting on a seismically violent set of faults, took that power very seriously. His association with the horse is equally deep, he is Hippios, and myth makes him the sire of horses and of the winged Pegasus. Sea, earthquake and horse form a single older complex of a god of the surging, uncontainable forces beneath and around the land.
Linear B confirms his antiquity and something surprising about his rank. In the Mycenaean tablets from Pylos, written before 1200 BCE, the name po-se-da-o appears prominently, at that palace apparently more often than Zeus, receiving offerings alongside a pair of goddesses. Whatever the exact hierarchy, Poseidon was a major deity of the Bronze Age mainland, not a late specialist of the waves.
By the classical period his marine role dominated, fitting a seafaring people whose ships he could wreck or spare. Sailors sacrificed to him before a voyage; his anger, in the Odyssey, is what keeps a man from home for ten years.