Greek Mythology
In Greek mythology Cerberus is the name given to the three headed watch-dog who guarded the door to Hades, he would allow only for souls to enter the Underworld, no soul could exit. Cerberus is the child of Echidna and the Giant Typhoon. Cerberus was a hound belonging to Hades brought down when captured by Heracles for his final of the twelve labors.
 God Of War franchise
Kratos encounters many forms of Cerberus throughout the God Of War series however it is in God Of War 2 when Kratos battles the real Cerberus and kills it so that he could retrieve the Golden Fleece.