The origin of Arion is slightly varied. In one account, Arion is the son of Poseidon (the Greek god of the sea) and Demeter (the Greek goddess of the harvest). According to Theoi, a Greek mythology website, Poseidon had been pursuing Demeter. Demeter, in an effort to escape Poseidon, transformed into a mare. Poseidon, in turn, transformed into a horse. Poseidon tricked Demeter into mating, and Demeter later gave birth to Arion. Other accounts claim Arion came from Gaea, the Greek earth goddess, or that he was created by Poseidon in a contest with Athena (the Greek goddess of wisdom).
In each account of Arion's birth, he is the creation of some divine being in Greek mythology. Greek literature describing the horse claims that he is from "divine origin" and is therefore an immortal. The horse is therefore unable to die. Greek mythology also claims that Arion helped many heroes. According to Theoi, Hercules rode Arion into battle; Arion also raced in the chariot race of the first Nemean Games.
Arion is known for his speed and the ability to fly. In book 23 of the Illiad, Homer makes a reference to Arion's speed during a chariot race, essentially saying that one of the racers could not be beat even if those behind him were riding Arion. In other accounts, Arion is described as being fast, but light on his feet; he is able to run across water without getting wet or run over the tops of corn ears without breaking them.
Descriptions of Arion vary by source. Many accounts describe him as a black horse with wings; some accounts, such as Statius' "Theibaid," describe his mane as "fiery red." Because many versions of Arion's story indicate that he was begotten from a horse and a human figure, he is said to have human feet on one side of his body.