Ever had a Jack Russell Terrier? Those little guys will run circles around all kinds of other dogs and animals. Their human better be healthy and active by taking the Jack’s out everyday to run and play because if they don’t; they get neurotic. They aren’t the only breed like that either.
I think the important takeaway on this subject is when people decide to get a pet for their home to be a family dog; they need to do their research to make an informed decision. Especially with purebreds. All of mine are rescues and none of them purebreds. Another thing that tends to calm dogs down, even the more athletic active ones, is age. I like to get rescues that are at least 3 years old. They have all that puppy behavior (chewing up shoes and stuff) out of their system and they still listen/learn well. Once they get to a certain age which varies by breed; the story about teaching old dogs new tricks is true. Hopefully their new human can accept them the way they are. Old dogs don’t usually want much anyway. Some attention and affection. Some treats and of course their food and fresh water.
People can post a million studies about this breed or that breed on FR till the cows come home. Nothing is accomplished and it has nothing to do with what this forum is all about but you know what they say about teaching old dogs new tricks...
I have spent time around Jack Russell Terriers, and am familiar with the breed.
Terriers of all types (dachshunds are another terrier) are historically bred to kill vermin - rats, mice, rabbits, etc, usually in a farm or rural setting, although they are being used by city dwellers in NY to address the rat problem. It is interesting to watch them work. I would not keep one in the same household with a bird, rabbit, cat, or ferret.
But Jack Russells are not and have not been bred to dogfight. Therein lies the difference.