Even though the Jack Russell Terrier is a small dog, they have a huge personality. So big, in fact, that without the proper leadership they will become unruly and rambunctious. They are incredibly bright, loving, and loyal dogs and they are great with kids so long as the kids play nice, but they should not be left alone with other, smaller pets. If they are allowed to think that they are pack leader, this could result in certain detrimental behaviors, such as snapping, excessive barking, and guarding areas they believe to be ‘theirs.’ They also require a fair amount of exercise, otherwise they might become restless. .