A bull whippet, or bully whippet, is a purebred whippet that has an inherited mutation in the myostatin gene. Whippets that inherit one copy of the mutation look normal but run faster than regular whippets. Whippets that inherit two copies of the mutation are bully whippets with excessive muscling.
Bully whippets have broad chests and bulging muscles. The heavy build is similar to that seen in the pit bull and the American bully, explaining the name. The mutation in the myostatin gene disables the gene. When the gene cannot function normally, excessive muscle development occurs. Dogs with two mutated genes have almost double the amount of muscle tissue found in a normal dog.