Down to some Greek Mythology on Orthros, Orthros was a two-headed, serpent-tailed dog, a brother of monsters such as Kerberos and the Khimaira better known as Cereberus and Chimera. He mated with the latter, siring the Sphinx and Nemean Lion. (Wow, diden realise mythological beasts can mate!) His master was the three-bodied giant Geryon, king of the sunset isle of Erytheia. Orthros was set to guard his master's fabulous herd of red-skinned cattle. That being said, this info has totally nothing to do with the series, minus the fact that Ryosuke put a dog soft toy beside his student's bed and orthros might appear somewhere later in the movie??