A Jack Russell terrier is a small but feisty dog, and you probably know this if you already have one. It can be a loving pet, but, at the same time, a true house protector being very territorial in its nature, energetic, fast, loyal, smart and let’s not forget LOUD.  They were originally hunting dogs, and they can also be potentially aggressive in some cases. Even though people don’t think of Jack Russells as guard dogs per se, this is a breed is very trainable in this way, due to some of these inborn characteristics.