Arion 
 
Immortal horse. Arion was the offspring of Poseidon and Demeter. Poseidon lusted and chased after his sister Demeter, not long after Persephone's abduction. Demeter tried to hide from her brother, by changing herself into a mare, hiding among the other horses in Arcadia. Nevertheless, Poseidon found her, transformed himself into a stallion and mounted her. Demeter gave birth to a girl named Desponia, and the colt known as Arion.

Arion was owned by Oncius in Arcadia. Arion was fabulously swifter than any other horses, which Heracles won from Oncius. However Heracles gave up Arion to Adrastus, the king of Argos.

In the war of the Seven Against Thebes, when Theban defeated the Argives, Arion saved Adrastus' life by carrying the king from the battlefield, and brought him safely to Athens