When I was a kid we had a Jack Russel the thing was mad, when I let it off the lead it would seek out the biggest dog in the park and launch itself at it, usually around the throat. Dobermans and Rottweilers were his favorites, in the end I couldn't let him off the lead because I knew if he got it wrong it would have been game over. He loved people and kids though, a great pet for a boy of 11.