“Throughout the years since the bombs first fell, dogs have remained largely unaltered by radiation.”— Fallout 3 loading screen
Some dogs have remained domesticated pets, but many more have turned feral and hunt the dusty wasteland in packs. The dogs that survived the Great War are a larger, and tougher, breed - now almost wolf-sized. You won't encounter any rabid toy poodles in your travels. Even so, just as they have in ages past, many dogs have remained faithful and beloved companions to men and women. Inarguably, the most famous of these post-nuclear canines were Dogmeat, the companion of the Vault Dweller, and his descendant, the companion of the Lone Wanderer.
Though most breeds of dogs remain largely unaffected by radiation exposure, some modified or mutant varieties do exist. The centaurs are mutants created by mixing dog, human, and brahmin DNA in FEV vats. Dogs were also subject to Enclave experiments, which resulted in cyberdogs. Australian cattle dogs survived the war, and German Shepherds lived on in the Commonwealth, as well as through cyberdogs. A Siberian Husky named Sasha was alive in 2161. Some pure-breed dogs, such as the yellow dog, also survived. New breeds also emerged, such as the common wasteland dog and Legion mongrels native to Denver. Most dogs seen around the Commonwealth as well as Appalachia have become vicious wild cures known as mongrels.
Legion mongrels are dogs owned and bred by the Houndmasters of Caesar's Legion. Mongrels are mainly used in combat and scouting missions by the Legion. Because of their thick, shaggy hair (as seen in the picture) and their red-colored eyes, they are of a distinctly different breed than most dogs found in games.
Vicious dogs are a nearly hairless and apparently, partially ghoulified species of wasteland canine with a close similarity to Pitbulls. Ruthless survivors of the wastes, these starving, irradiated mutts will attack anything that moves.