The two-headed dog that guarded the cattle of Geryon was Orthrus. He was a monstrous watchdog defeated by Heracles during the hero’s tenth labor.
Orthrus guarded Geryon’s red cattle on the western island of Erytheia, together with the herdsman Eurytion. In the account attributed to Apollodorus, Heracles killed Orthrus with a club before defeating Eurytion and Geryon and driving the cattle away.
Orthrus was traditionally the offspring of Typhon and Echidna, making him the brother of Cerberus, the better-known hound of the underworld. This family connection is one reason the two creatures are often confused. Cerberus guarded Hades, whereas Orthrus guarded livestock belonging to the giant Geryon.
The name also appears as Orthus or Orthos in ancient sources. Hesiod uses “Orthus,” while Apollodorus uses “Orthrus,” the form most common in modern English reference works. Artistic depictions vary: although two heads are standard, some ancient images show one or even three heads.