Cerberus

Cerberus (地獄の番犬ケルベロスJigoku no Banken Keruberosu) is the terrible three-headed watchdog from Hell. It's name means demon of the pit. In Greek mythology he is Hades' most beloved dog. It appears in vol.23 of Kurumada's manga, devouring souls in the second prison of the Underworld. The Specter, Sphinx Pharaoh, was fond of the creature and took care of him. Kerberos was defeated, although not killed, by Pegasus Seiya and Andromeda Shun, to be able to proceed further into the Underworld.

Mythology
In Greek Mythology, Cerberus was the three-headed dog that guards the gates to the Underworld. He was the son of the demon Typhon and his mate, Echidna who were created by Gaia to defeat Zeus as revenge for his imprisonment of the Titans in Tartarus. After Typhon was imprisoned beneath Mount Etna, Zeus let Echidna's offspring live so that they may present a challenge for future heroes. Contrary to Dante and Kurumada's depictions of him, Cerberus has an appetite for live meat, he is friendly to any dead soul who enters the underworld, but if they try to escape then he gets angered. The only way to calm him down is with either drugged honey cakes, which is what the Cumaean Sybill did when she helped Aeneas venture through the underworld, or with music as with Orpheus did with his enchanted lyre. Hercules, on the other hand, used his bare strength to bring Cerberus up to the surface on his 12th and final labor, after which he returned the dog to Hades as he promised.