In addition to his more familiar role as lord of the sea, Poseidon was also the god of horses. He sometimes adopted the form of a horse when pursuing mortal women, which may account for the fact that some of his children were horses, including Arion, one of the steeds of Achilles. It is not known if Poseidon appeared as a horse when he seduced Medusa, but when she was killed, she produced the winged horse, Pegasus.
"Lord Poseidon, from you this pride is ours. The strong horses, the young horses, and also the rule of the deep." – source Unknown