Dogs are creatures that appear or have been set to appear in every single Fallout game. Unlike other animals after the bombs dropped, Dogs have largely not been affected by radiation and other virus's like FEV, and stay loyal to their masters (i.e a dog with a Scavenger). The player character comes in direct contact with dogs in all of the games, a notable dog is Dogmeat, a dog in all three Fallout games, and can be recruited by the Vault Dweller, the Chosen One, and the Lone Wanderer. It has also been confirmed that dogmeat will return in Fallout: New Vegas and the Courier will have the option to recruit him.
Although dogs are unaffected by radiation, they can still be hostile in the Wasteland. Wild Dogs (Fallout, Fallout 2) and Vicious Dogs (Fallout 3) are a good example. Vicious/Wild dogs will attack the player on sight and will do anything for food.
 Fallout and Fallout 2
Like stated before, Wild Dogs were common in the West in these games. They usually knock the player down.
 Fallout 3
In Fallout 3 dogs are usually found following Scavengers. All tame Dogs are non-hostile creatures in this game, they will only become hostile if they are attacked or you attack the Scavenger they were following.
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