The Bullmastiff is a large-sized breed of domestic dog, with a solid build and a short muzzle. The Bullmastiff is mastiff type dog, and was originally developed by 19th-century gamekeepers to guard estates. The breed was created by crossing the English Mastiff with the. now extinct Old English Bulldog. It was recognized as a purebred dog by the English Kennel Club in 1924. By using this site, you agree to the Terms of Use and Privacy Policy.