Devil Dog is a mutant dog worshiped by the Blackfoots. The Hangdogs want you to either tame or kill him, although they would rather see him killed. If you have a high Outdoorsman skill and tasty food, you can tame him. He is also sick with a rabies-like disease, making him aggressive and highly hostile. If you cure him before you tame him, he will wander away from Blackfoot territory. He may also be encountered later in the game.
If you tame him first before curing him, he gets sicker and sicker until he dies. The Hangdogs want to use him for breeding to have his bloodline in the Hangdogs' dogs. He has an evil spirit which brings bad luck to the tribe, which is why they call him the "Devil Dog" and would prefer him killed.
Devil Dog has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.