This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Orthrus (Greek)
In Greek mythology, Orthrus (Greek: Orthros) or Orthus (Greek: Orthos) was, according to the mythographer Apollodorus, a two-headed dog who guarded Geryon's cattle and was killed by Heracles. He was the offspring of the monsters Echidna and Typhon, and the brother of Cerberus, who was also a multi-headed guard dog.
According to Apollodorus, Orthrus had two heads; however, in art, the number varies. As in the Samos pectoral, Euphronios' cup, and the Swing Painter's, amphora, Orthrus is usually depicted with two heads,although, from the mid sixth century, he is sometimes depicted with only one head, while one early fifth century BC Cypriot stone relief gives him three heads.