Cerberus

Cerberus is the Mister Gutsy model robot patrolling the ghoul city of Underworld. If the Lone Wanderer speaks to him, he or she will learn that he is not fond of his role in protecting the ghouls and that a combat inhibitor prevents him from killing everyone in sight. A player with the Robotics Expert perk can remove Cerberus's combat inhibitor, but Cerberus will become hostile to the player too.

Appearances
Cerberus appears only in Fallout 3.

References: in Greek Mythology, Cerberus is the 3-headed dog who guards Hades and the Gateway to Underworld.