Jack Russell Terriers. My ex had two during our marriage. I've seen them chase German Shepherds down the street. Ours were so well matched that till one died, there was a constant and frequently violent struggle for dominance.

And nikirtehsuxlol is right, that it is also the owners. Since I've had good dogs before the marriage I watched Ceasar to see what had I been doing wrong with the JRTs. I've come to understand where I went wrong, but also realized that even if I had been perfect, there was no way I could have overcome the influence of my ex. Still, if I get another dog it's going to be a Sheltie.