Some times in life animals just don't get along. In real life there are steps you can take to reduce the odds of this happening and there are steps that can help reduce or eliminate the conflict. But even then it isn't always successful. In sims, well, it happens there too, but in sims I don't think it is possible to repair the relationships of pets with each other. Good news is the fights will never be fatal or cause them not to eat, unlike real life. My advice is move one of the pets into a new home. I had faced this issue and it is what I did. I wish I had better advice. Does anyone else have a better workaround?