My next door neighbours sons Staffy used to always come over to play with my other dog and normally gets on with any other dog. Recently the Staffy came and saw my new Boarder Terrier pup and didn't like him at all she growled at him for no reason. He did nothing at all to her he was in my mums arms the Staffy was over other side of fence. Could it be just that she doesn't understand why my new pup is here instead of my other dog that unfortunally I lost in car accident. In the Staffys eyes it would seem like the one she played with just disappeared and got replaced with this new pup. I'm thinking its because of not understanding where my other dog is why she growled at my pup as they got another Staffy the 1st 1s daughter now fully grown and that seemed fine with my pup though didn't get close but showed no aggression and that one never met my other dog.