A mastiff is a large breed of dog. They are often used as guard dogs and there are many breeds:
Alpine Mastiff, an extinct breed originating in Switzerland
American Mastiff, a breed of dog originating in the United States
Anatolian Mastiff, an ancient breed of dog originating in Turkey (when it was part of the Persian Empire in 485 AD)—see Anatolian Shepherd Dog
Argentinian Mastiff, or Dogo Argentino a mastiff developed in Argentina for large game-hunting
Bordeaux Mastiff or French Mastiff, two alternative names for the Dogue de Bordeaux, one of the most ancient French breeds
Boxer (dog) or german bullmastiff, bred to be a lighter faster stronger bullmastiff
Brazilian Mastiff, another name for the Fila Brasileiro a breed of dog originating in Brazil
Bullmastiff, said to be a cross between the English Mastiff and the Bulldog
Bully Kutta, Pakistani Bully Kutta
Cane Corso Mastiff, or Italian Mastiff, breed which originated in Italy and best known as the Cane Corso
Cuban Mastiff, an extinct breed resembling a Bullmastiff, originating in Cuba
English Mastiff, a breed of dog in the Molosser group originating in England, usually just called "Mastiff"
German Mastiff, better known as Great Dane, a breed of dog originating in Germany
Japanese Mastiff, also known as Tosa Inu, bred as Japanese fighting dogs
Korean Mastiff, also known as Dosa Inu, a breed of Korea bred from Tosa Inu and European breeds
Neapolitan Mastiff, an ancient breed of dog originating in Italy
Gran Mastín de Borínquen, also known as the Puerto Rican Mastiff, a breed of dog originating in Puerto Rico
Pyrenean Mastiff, a guardian breed of the Spanish Pyrenees
South African Mastiff, also known as Boerboel, a breed of dog originating in South Africa
Spanish Mastiff, an ancient breed of dog originating in Spain
Tibetan Mastiff, an ancient breed of dog originating in Tibet
