Here is a list of mythical creatures beginning with this letter and I am constantly gathering information from each continent and continually updating.
Arion (Greek)
In Greek mythology, Arion or Areion, is a divinely-bred, fabulously fast, black-maned horse. He saved the life of Adrastus, king of Argos, during the war of the Seven against Thebes. Arion was the offspring of Poseidon and Demeter. When the goddess Demeter was searching for her daughter Persephone, she was pursued by Poseidon. To escape Poseidon, Demeter turned herself into a mare and hid among the mares of Oncius, king of Thelpusa in Arcadia.
Arion was given to the hero Heracles, who rode Arion into battle during his expedition to Elis, and also during his combat with Ares' son Cycnus. Arion is mentioned as early as in the Iliad of Homer, where he is described as the "swift horse of Adrastus, that was of heavenly stock." A scholiast on this line of the Iliad explains that Arion was the offspring of Poseidon.