In most stories, Areion was conceived while the goddess Demeter hid from her brother Poseidon under the guise of a mare. Aerion was a result of Poseidon seeing through the ruse. Other sources say that Areion's mother was Gaia, the goddess of the earth itself. Others claim Areion was sired by Zephyrus, one of the four wind-gods, and that its mother was a Harpy.
Heracles stole Areion from Oncius, then rode it in battle against the Eleans. After the war, he gave the immortal horse to Adrastus.
Areion is mentioned by the ancient poets Homer and Sextus Propertius, and the geographer Pausanias.
"ARION : Immortal horse of Adrastus of Argos ; Greek mythology : AREION." ARION : Immortal horse of Adrastus of Argos ; Greek mythology : AREION. N.p., n.d. Web. 14 June 2014. (http://www.theoi.com/Ther/HipposAreion.html).
"Areion." Wikipedia. Wikimedia Foundation, 27 Feb. 2014. Web. 14 June 2014. (http://en.wikipedia.org/wiki/Areion).
"Arion" Theoi Project: Greek Mythology. (http://www.theoi.com/Ther/HipposAreion.html)
"Arion" GreekMythology.com (https://www.greekmythology.com/Myths/Creatures/Arion/arion.html)