Cerberus

Cerberus is an enormous semi-transparent three-headed rottweiler found at the gates to the Underworld. The dog gets clearer the closer you are to death, relitivly playful in nature never acutuly attacked Percy. One of the labors of Hercules.

The Lightning Thief
In The Lightning Thief, Percy, Grover, and Annabeth encounter Cerberus and Annabeth manages to "tame" him by using a large red ball and some skills she learned at obedience school. Annabeth plays with Cerberus using the red rubber ball while Percy and Grover goes past him. After Cerberus chomped the rubber ball, Annabeth tells him that they will play again someday and goes through.

Later when speaking with Hades, Percy mentions to him that he should play with Cerberus more and that he likes big red balls.

The Last Olympian
In The Last Olympian, Mrs. O'Leary plays with Cerberus when Percy and Nico are in the Underworld trying to follow Luke's childhood.