19: CHIHUAHUA

Considering the fact that the Chihuahua is the smallest known breed to man, there is a reason why it has found itself on this list. Originating from Mexico, and named after the state with the same name, these tiny little dogs are easily provoked, and therefore may attack just as quickly as they bark. The term “small but terrible” describes them perfectly as they have no patience for younger kids, and normally are loyal to only one person.