These are dogs that have undergone major mutation from being exposed to the Forced Evolutionary Virus. Usually they are accompanying super mutants. They may have been created in the FEV lab of the Institute, along with the rest of the super mutants that are seen throughout the Commonwealth.
Mutant hounds are usually several times bigger than normal stray dogs and mongrels. The modification imposed on their bodies is similar to the one the super mutants went through when they were humans: increase of muscle mass, depigmentation of the skin to a greenish color and complete loss of body hair. The intelligence of mutant hounds seems to be affected too in the same way that a super mutant's is from the Forced Evolutionary Virus. They appear not to be in any way linked to the Master, as he was evolving only humans for his army. Super mutants seem to understand mutant hounds like no other in the Commonwealth. Their vocal cords have evolved to create a very deep and low howl that they use to alert their nearby brethren.
Mutant hounds will alert nearby mutants using a very loud howl. They deal radiation damage upon biting you. They normally come in packs of 1-3. 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 looks as the basic varint but they have more hit points than usual, and can deal a decent amount of RADs with a bite. As such, they still emit a loud howl to alert other super mutants of a threat.