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 Cerberus would much rather "send [the ghouls] back to hell" than serve as their guard dog, but his combat inhibitor prevents him from slaughtering the "zombie freaks."

With the Robotics Expert perk, the player can remove Cerberus's combat inhibitor, resulting in negative karma and Cerberus's immediate hostility with the player and everyone else in Underworld. The residents of underworld won't stand a chance if you don't help them, sometimes they may be able defeat him but that case is really rare. A good idea if you don't want to fight him you can take some Stealth Boys or use the Chinese Stealth Armor and then, enjoy the show!

Or you could shoot the Combat Inhibitor and frenzie him with no loss of karma whatsoever, also when he is killed you get a little XP.

Appearances
Cerberus appears only in Fallout 3.

Trivia

 * In Greek Mythology, Cerberus is the 3-headed dog who guards Hades and the Gateway to Underworld. Through dialog, the player can get Cerberus to reluctantly agree that his job is, in essence, that of a "guard dog," protecting Underworld from any hostile intruders, which is analogous to the role of the myhtological creature. Also, just as the mythological Cerberus has three separate heads attached to one body, the Cerberus of the Fallout universe comparably sports three sensor modules to one chasis.

Quotes

 * "Atten-shun! Civilian on deck."
 * "Go Underworld! Go Ghouls! Yay!"
 * "...Damn this pansy zombie programming..."
 * "Closing dialog system."
 * "Personally I think they're a bunch of rotting zombie-maggot farms, and I'd sent them back to Hell if I could, DAMN THIS COMBAT INHIBITOR!"
 * "Whoa Nelly! Sensing Combat Inhibitor removal! Guns are hot! Firing for effect! Welcome to Hell, you zombie freaks!"
 * "Thank you" After destroying the combat Inhibitor.