How did Hades get Cerberus?

Cerberus was born from an egg. He was one of the offspring of Echidna (part woman, part serpent) and Typhon (a giant who was serpent to the hips with a human upper body, some depict the head as human, some as dragon). When Cerberus was still a pup, Hades called on him and his mother offering a gift of gold jewelery. Echidna stated that she would accept the gift if it were a gift for Cerberus's birth, but not if it was a payment to take the pup. Hades left the gift anyway as a show of what serving him could bring.

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 and no living human to enter the underworld. Delia's spirit would be given in return after his thousand years of service.