Cerberus is a three-headed dog in Greek mythology that guards the gates of the Underworld for Hades. It prevents the dead from leaving and the living from entering. Known for its terrifying power, Cerberus was famously subdued by Hercules during one of his labors.
source
according to many sources, he basically asked hades if he could take cerberus for a walk to meet a king, and hades accepted.
He also delivered Cerberus to Hades as one of his labor's.
It's not Hercules it's Heracles