Mutant hounds are usually several times bigger than normal stray dogs and mongrels. The modification imposed on their bodies by the Forced Evolutionary Virus is similar to the one the super mutants went through: increase of muscle mass, depigmentation of the skin to a greenish color and complete loss of body hair. They are approximately 3 feet tall at the shoulder, and 5 feet long. Their intelligence seems to be affected too in the same way that a super mutant's is from the FEV. Super mutants seem to understand mutant hounds like no other in the Commonwealth.
Mutant hounds will alert nearby mutants using a very loud howl. Their bite inflicts radiation damage. They normally attack in packs of up to three. They can also be tamed if one has the Animal Friend perk and be captured with Wasteland Workshop installed.
Mutant hounds are dogs that have been infected by FEV. They will not attack super mutants of any type, and will make a loud howl to alert other super mutants in the area. They are usually in packs of other super mutants.
Glowing mutant hounds share the same appearance as the basic variant but they have more hit points than their regular counterpart, and can deal a decent amount of radiation damage with a bite. As such, they still emit a loud howl to alert other super mutants of a threat.
↑Fallout 4 loading screens: "Super Mutants are often accompanied by Mutant Hounds, monstrous dogs that are just as dangerous as their masters."
↑Fallout 4 Vault Dweller's Survival Guide p. 44: "Super Mutants are massive, muscle-bound creatures with a natural immunity to radiation damage and an inherent hatred of humans. Most Super Mutants carry fairly crude gear, but they do have their share of powerful weapons, and many Super Mutant groups use trained Mutant Hounds to chase down trespassers. Super Mutants aren't particularly intelligent, but their physical strength and natural aggression have allowed them to establish a significant presence throughout the Commonwealth."
↑Fallout 4 Vault Dweller's Survival Guide p. 44: "Compared to a Super Mutant's lumbering gait, Mutant Hounds move very quickly. They also have powerful jaws and an impressive amount of health. Aim for the head to take them out as quickly as possible, or focus your fire on their legs to slow them down."