The Mastiff is the former leader of D.O.G. HQ.
Not much is said about the Mastiff. He seems to be a strong leader for the dogs. He is seen decorated with honors and medals in the first film. Apparently, he has had some adventures of his own.
The Mastiff is later replaced by Lou.
The Mastiff is first seen when a small dog brings him a bone shaped capsule filled with papers reporting that former agent guarding the Brody house hold has been taken by the cats, though the dog is safe he had to be replaced as soon as possible this where Lou comes in. A doberman was supposed to get Lou's job, but all did not go as planned.
The mastiff is seen again when the delegates of the dogs are fighting about what to do after Lou's family gets kidnapped by the cats.
The Mastiff doesn't appear in the film. Instead, Lou takes his place.