Orthus (also Orthrus and Orthos) was a two-headed dog and a doublet ("brother") of Cerberus in Greek mythology. Both were whelped by the chthonic monsters Echidna and Typhon.
He was owned by the three-bodied giant, Geryon. Orthrus and his master, Eurytion, were charged with guarding Geryon's herd of red cattle in the "sunset" land of Erytheia ("red one"), one of the islands of the Hesperides in the far west of the Mediterranean. Heracles eventually slew Orthrus, Eurytion, and Geryon, before taking the red cattle to complete his tenth labour.
Orthrus was one among Echidna's fearsome brood listed in Hesiod's Theogony. According to some sources, it was he rather than Typhon that sired, with Echidna, further chthonic monstrous creatures: the Chimera, the Sphinx, the Lernaean Hydra, and even, Hesiod says, the Nemean Lion, and Cerberus.