In Greek mythology, Orthrus was a two-headed dog and one of the many monsters sired by Typhon and Echidna, among his siblings were Cerberus, Sphinx, and Chimera to name but a few. Orthrus was the dog of the Titan, Geryon, and along with Eurytion was the guardian of Geryon's herd of red cattle. Geryon, Eurytion, and Orthrus were all slain when Hercules ('Heracles' in Greek mythology) was completing the tenth of his twelve labors.
Orthrus appears as a servant of Pazuzu, and worked alongside the Partner before she broke away from Pazuzu's control. After reaching the ruins of Tokyo, Pazuzu instructs the hero and his friend to meet up with Orthrus at the Princess Hotel and assist him in taking out the Partner. During their confrontation with her at Tokyo Tower, the hero ends up siding with her and the friend and Orthrus part ways with him. Orthrus continues to fight alongside the friend after his transformation into the Dark Hero, but is killed in battle with the hero's party.
"A gigantic, two-headed dog, the child of the storm giant Typhon and the half-snake beast Echidna. His body is as big as an ox, and he has the fearlessness and ferocity of a lion. Orthrus guarded the underworld alongside Cerberus, but was also said to guard the herds of the monster Geryon, who lived on the island of Erytheia at the edge of the world. However, he was killed by a blow from the club of Heracles, who came to the island as part of his Twelve Labors. He also fathered the Sphinx, the Nemean Lion, and other monsters with his mother Echidna."
—Shin Megami Tensei IMAGINE Compendium
Orthrus appears on Suginami field and within the gold level instance of Suginami Tunnels and the Nakano Stone Site. He can be contracted during the daytime in Ikebukuro outside the gates of Zoushigiya Cemetery. He appears regularly as a field boss in Suginami and alongside Cerberus and Garm as the final bosses of the Suginami Tunnels Unknown boss rush. He is also one of the mountable demons within the game.
"The two-headed dog who, along with his master Eurytion, protected the Titan Geryon's red cattle in Greek lore. When Hercules went to claim Geryon's cattle for his tenth labor, he slew Orthrus with one swing of his club."
—Devil Summoner: Soul Hackers 3DS Compendium
Devil Summoner: Raidou Kuzunoha vs. The Soulless ArmyEdit
"The two-headed dog who protected the titan Geryon`s red cattle in Greek mythology. Despite his prowess as a guard, Hercules killed him in one blow while performing his Twelve Labors."
"The two-headed dog who appears in Greek lore as the younger brother of Cerberus. He protected the titan Geryon's red cattle, but despite his skill as a guard, Hercules killed him in one blow while performing his Twelve Labors."