Through the 200 years since the Great War, dogs have remained unaltered by radiation, seemingly a miracle. Other than that, there is no reason to believe that the biology of dogs in the Fallout universe differs from dogs in the real world, besides having a rougher appearance than domesticated dogs, unsurprising in the harshness of the Wasteland.
Legion mongrels and NCR guard dogs will attack players that have low reputation with their associated faction. Fiend guard dogs and vicious dogs attack on sight, regardless of reputation, while wild dogs and coyotes will only attack if the player is close enough to make them feel threatened. With the Animal Friend perk, dogs will not attack unless provoked.
Regular dogs with standard statistics and characteristics but no special attributes. Large dogs don't differ much, aside from being stronger than the basic variety. Regular dogs roam Zion Canyon. Old Lady Gibson owns 6 of these animals to guard her junkyard.
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. The current mother of the mongrels in The Fort is Lupa. 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 the game. Their appearance more resembles that of a wolf or coyote. While most of them are bred, Antony says the Legion sometimes finds good mongrels in Denver, the Dog City.
The breed commonly used as guard dogs looks similar to the vicious dogs that roam the Mojave Wasteland. As is with the vicious dog, the NCR guard dogs are not as resilient as the Legion mongrels. This is probably due to their lack of selective breeding and decent training, unlike the Legion who regularly train their dogs in fights against captured enemies in The Fort and only select the strongest dogs to breed.
Vicious dogs are a nearly hairless species of wasteland canine with a close similarity to pit bulls. Ruthless survivors of the wastes, these starving, irradiated mutts will attack anything that moves, including you, unless you have acquired the Animal Friendperk. Vicious dogs are also known to hunt in packs.
The vicious dogs in Fallout: New Vegas resemble the ones featured in Fallout 3, as well as the guard dogs used by the NCR, and Fiend guard dogs used by Violet.
Wild dogs and large wild dogs can be found near the NCRCF. They are rarely hostile and will only attack the player if they get too close, or if the player first provokes them by attacking them or one of their pack.
White Legs mongrels appear throughout the Zion Canyon, accompanying members of the White Legs tribe and will attack the player on sight. They are identical to the Legion mongrel, which implies that Caesar may have given them to the tribe to aid in destroying New Canaan. White Legs mongrels will not attack on sight with the Animal Friend perk.