Bull is a male American Pit Bull and is the main antagonist of Lassie.
Bull has no respect for other dogs, or for any humans. He and his pack live off the city waste and what they can scavenge nearby.
Bull was first seen trying to catch Lassie at the beginning of the story but was saved by German much to his annoyance. Bull at one point (along with his pack) attack some adults and children at a school. Bull escapes, and Lassie later comes to him to warn him that the humans were after him. Despite this, Bull does not listen and attacks Lassie. Bull was then caught by the dog-catchers, and last seen being driven away in a van by animal control to be put down.