Cerberus

In Greek mythology, Kerberos ("demon of the pit"), often called by the Latinized name Cerberus, was the hound of Hades --- a monstrous three-headed dog (sometimes said to have fifty or one-hundred heads), (sometimes) with a snake for a tail and innumerable snake heads on his back.

He guarded the gate to Hades (the Greek underworld) and ensured that the dead could not leave and the living could not enter. His brother was Orthrus.

Cerberus is the offspring of Echidna and Typhon.

He was overcome several times

1. Heracles' final labour was to capture Cerberus. First, Heracles went to Eleusis to be initiated into the Eleusinian mysteries. He did this to absolve himself of guilt for killing the centaurs and to learn how to enter and exit the underworld alive. He found the entrance to the underwold at Tanaerum. Athena and Hermes helped him through and back from Hades. Heracles asked Hades for permission to take Cerberus. Hades agreed as long as Heracles didn't harm him, though in some versions, Heracles shot Hades with an arrow. When Heracles dragged the dog out of Hades, he passed through the cavern Acherusia.
2. Orpheus used his musical skills to lull Cerberus to sleep.
3. In Roman mythology, Aeneas lulled Cerberus to sleep with drugged honeycakes.
4. In Roman mythology, Psyche also lulled Cerberus to sleep with drugged honeycakes.

He can be found also in Dante's Divine Comedy, in Canto VI of Inferno (third circle)