5. Tibetan mastiff
Majestic, large and highly protective, Tibetan mastiffs have very firm ideas about their territory, which often extends past their owner’s property lines. This protective instinct came in very useful during their long history as livestock guardians, but it can be problematic in suburban areas. The breed is intelligent and very independent, like many livestock guardians, which means that they often make their own decisions about what is safe for their human families. This makes them potentially dangerous without serious training and can easily translate into territorial aggression.