In Greek mythology, Orthrus or Orthus 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.

In Greek mythology, Geryon, son of Chrysaor and Callirrhoe, the grandson of Medusa and the nephew of Pegasus, was a fearsome giant who dwelt on the island Erytheia of the mythic Hesperides in the far west of the Mediterranean. A more literal-minded later generation of Greeks associated the region with Tartessos in southern Iberia. Geryon was often described as a monster with either three bodies and three heads, or three heads and one body, or three bodies and one head. He is commonly accepted as being mostly humanoid, with some distinguishing features and in mythology, famed for his cattle.