Through the 200 years since the Great War, dogs have remained unaltered by radiation. Because of that, there is no reason to believe that the biology of dogs in the Fallout universe differs from dogs in the real world, other than having a rougher appearance than domesticated dogs which is unsurprising due to the harshness of the Wasteland.
Legion mongrels and NCR guard dogs will attack player characters that have low reputation with their associated faction. Fiend guard dogs and vicious dogs will attack on sight, regardless of reputation, while wild dogs and coyotes will only attack if the player character 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 do not differ significantly, aside from being stronger than the basic variety. Regular dogs roam Zion Canyon. Old Lady Gibson owns six dogs 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. Due to their thick, shaggy hair and their red-colored eyes, they are a distinctly different breed than most dogs found in the game. Their appearance more-so resembles a wolf or coyote. While most of the mongrels are bred, Antony informs the Courier that some mongrels come from 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 training.
Vicious dogs are a nearly hairless species of wasteland canine. Ruthless survivors of the wastes, these starving, irradiated mutts will attack anything that moves, including the player character, unless they have acquired the Animal Friend perk. 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.
Vicious dog 0001CFA3
Very aggressive: Will attack enemies and neutrals on sight.
Wild dogs and large wild dogs can be found near the NCR Correctional Facility. They are rarely hostile and will only attack the player character if they get too close, or if first provoked by attacking them or one of their pack.
Wild dog 0015C690
Aggressive: Will attack enemies on sight.
Average: Will avoid/flee from threats stronger than itself.
White Legs mongrels appear throughout the Zion Canyon, accompanying members of the White Legs tribe and will attack the player character on sight. They are identical to the Legion mongrel albeit slightly larger, 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.