Cerberus

He is a dog called Cerberus (can-cerbero) of enormous size and with three heads. His mission was to guard the gate of Hades, the underworld in Greek mythology. Cerberus carried out his work with great zeal, which is why his name is frequently used to refer to the work of any guard who performs his function with great seriousness.

In ancient texts, it is said that he greeted the newly arrived with his serpent-shaped tail and that he attacked ferociously anyone who attempted to flee.