Since Poseidon is well known as the god of the God of the sea, storms, earthquakes and horses? What is the connection between Poseidon and horses? As a god of the sea the storms and earthquakes part makes sense but where does the part of the horse come in?
Sailors prayed to Poseidon for a safe voyage, sometimes drowning horses as a sacrifice; in this way, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climactic battle of Issus, and resorted to prayers, "invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves." Papyrus Oxyrrhincus FGH 148, 44, col. 2; quoted by Robin Lane Fox, Alexander the Great (1973) 1986:168 and note. Alexander also invoked other sea deities: Thetis, mother of his hero Achilles, Nereus and the Nereids
Like Poseidon, Neptune was worshipped by the Romans also as a god of horses, under the name Neptunus Equester, a patron of horse-racing.