Orthus is a two headed monster dog and the brother of Cerberus. He appears in The Battle of the Labyrinth under the ownership of Eurytion. He also appears in Percy Jackson's Greek Heroes, where he is killed by Hercules.
Orthus had reformed along with Geryon. Although his official master is Geryon, he tends to listen more to Eurytion as he takes care of him, unlike Geryon. Similar to the ancient myths (Hercules), Percy Jackson, came to the ranch and killed Geryon, but unlike Hercules, he spared Orthus as he did not participate in defending his master because Eurytion had told him to stay back during the fight between Percy and Geryon.