Black Russian Terrier
The Black Russian Terrier is a large and powerful dog. It was originally bred to be used as guard dogs, herders and even sled dogs. Because of this, they're really muscular and energetic. Because of their background, Black Russian Terriers are great toward family but can be protective and wary toward strangers.
It requires daily exercise to keep fit and should be easily trainable. It can grow to about 26-30 inches.