My 75 lb GSD mix, Lucy, is an absolutely darling dog. But...a few months ago, my little 20 lb miniature Brussels Griffin, Pippa, discovered that there was a rat on the back porch and started hunting it. Lucy joined in. They never found it, but one night, something startled Pippa and she ran behind the couch. When she popped out the other end, Lucy grabbed her by the neck and started shaking her. It's a horrifying thing to witness. I think she thought that Pippa was what they were hunting. Since then, she is absolutely obsessed with Pippa, just adores her. We have to supervise carefully. Tonight, twice, she was loving on Pippa and suddenly grabbed her by the neck and started shaking her. She has 2 pretty bad bites on the back of her neck. We have a vet appt in the morning. We never know what will set Lucy off, so we can't prepare for it. We also have the same problem with our 75 lb GSD mix, Toby. When one of the other 3 dogs bumps him or startles him, he attacks. He's so much better now, will turn loose and run when we holler at him. Again, we never know what will set him off. Needless to say, the bigs and the littles are never alone unsupervised. They are crated when we leave. The bigs aren't being malicious when they attack, but I'm so afraid that one day, one of the littles will be seriously injured or killed. The vet suggested a dog behavioral therapist, but the ones he recommended want $1000 just to evaluate one dog. And since we never know what will precipitate an attack, I don't think a DBT can get a good reading on them. Our only recourse is to rehome the bigs, and I don't want to do that. Thinking about muzzling both of them!