Cerberus was a gigantic dog that guarded the gates of hell. This hound was the pet of Hades, lord of the underworld. Cerberus was described as a three headed dog with a serpent tail. It is also said he has a mane made up of snakes and lion claws. 

    Cerberus, Hades' pet, guarded the gates of hell. This beast made sure no ghost of the dead would leave. He also played a role in Hercules' twelve labors. Capturing Cerberus was Hercules' 12th and last labor. Hercules asked Hades if he can barrow Cerberus, but in order to do that Hercules had to first wrestle with the wild beast.

    It is said that one day Cerberus entered into a fight with a shark. When he came face to face with the shark, he saw it had a boat with a frightened girl in it. Cerberus killed the shark and pulled the boat away saving the girl who's name was Delia. Delia accepted Cerberus as a friend despite his frightful looks. One day when they were out together while Delia healed animals, a female approached persuading Cerberus to go see his mother. While Cerberus was gone, Delia was killed by Argus, a monster sent by Hades. Hades had constructed the whole elaborate scheme.
Cerberus was pained by the death of Delia, and struck a deal to serve Hades for a thousand years as a guard, allowing no soul to escape. Delia's spirit would be given in return after his thousand years of service. That is how Hades got a hold of Cerberus.