The history of the dog that did this was the I got her from the shelter at 7 months old. No "bad" experiences...she was just a puppy with puppy behavior and got to be too much for the previous owner. For instance, she was jumping on their toddler. She was a super good dog when we brought her home. We did do dog obedience with her and it was there that I found that she didn't overly care for other dogs. I had an older dog still at the time (about 14 years old) and all was well there. The older dog eventually passed after about 8 months. I waited a few a months and then got a JRT puppy (3 months old) from a breeder.
Things seemed to be fine. As the JRT got a little older (and very aggressive as JRTs are), there were squirmishes; mainly over toys or if the JRT was getting out of hand. The bigger dog would often try to "keep her in line" out on a walk by nipping an ear or something like that. One time when we were on vacation and my parents were taking care of them, they got into a fight over some food. Not sure who started it. So, there were a few incidents like that.
One day, they were out in the yard and I was in the house cleaning. I heard some barking going on and I checked out the window and they were both barking at something on the other side of the fence. My yard is large and heavily wooded and it was either a fox or a squirrel or something. Went and took a phone call for about 10 minutes. I then looked out my kitchen window and saw the JRT laying in the yard. I knew immediately something was wrong as the dog never sat still for anything. Ran out, grabbed her, and jumped in the car. She died on my lap on the way to the vet. When I got home, the bigger dog was bleeding from her chest and had a big "rip" in her fur/skin on her chest from the JRT.
They got into a fight or a frenzy of some sort. No way to say who started it. The behaviorist thinks they may have been both overexcited by whatever they saw through the fence and the "prey drive" kicked in for one or both. She said it was very common for dogs to have this happen when prey close by and they get wound up. Dogs will often get into it with each other. Due to the size differences of the dogs, the JRT didn't stand a chance.
After it happened, I immediately consulted with my vet about whether my "hound" was a dangerous dog and, based on the circumstances, I think he got a little upset with me that I might euthanize the dog when it was just "acting like a dog would around other animals." He was our trusted vet of 20 years so I had no reason to think he was being biased or anything. I know if he thought the dog was dangerous he would have said so. I then took it a step further and consulted with the behaviorist. They found her to be extremely good with people, and not overly fond of other dogs. Her prey drive would also kick in very easily when a small animal was in her periphery.