Cerberus

Basic Information
 is a three headed hell hound, traditionally known for being Hades' pet dog and guard dog for the underworld.

It was the offspring of Echidna, a half human half snake and Typhoon, a fire breathing giant (which even the olympians feared). His brothers were Hydra and Chimera and his half brother is suposedly Orthrus the two headed hell hound.

History
Greek Mythology:  is depicted as a dog with three ferocious head and the tail of a snake. His task is to guard Hades, only allowing the dead to enter. Only a few people have ever gotten past Cerberus to see Hades.
 * Orpheus, the famed musician, sneaked into Hades by using his music to lull  to sleep.
 * Hercules, with the permission of Hades, gets  in a choke hold, knocks him out and kidnaps him.