Orthus

Orhtus is the dog of Eurytion in The Battle of the Labyrinth.

The Battle of the Labyrinth
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, not Geryon. Similar to ancient myths, a powerful demigod, Percy, came to the ranch and killed Geryon, but unlike Hercules he spared Orthus as he did not participate in defending his master, as Eurytion told him to stay back during the fighting between Percy and Geryon.