Hi everyone. My fiance and I recently adopted an 8 year old female purebred GSD rescue (60 lbs), who is not known to have any aggression issues with other animals or an overly traumatic past. We also have a 6 year old male pom-chi mix (about 11 lbs). We tested the two dogs together and everything seemed fine. After a two week trial, in which there was one minor incident when the pom-chi got in the GSD's face and the GSD warned him to back off, there was no aggression and the two seemed to be getting on very well. We have watched them closely and do not leave them unmonitored. Last night, the GSD began to stare very intensely at our smaller dog. This morning, as I was laying down and she was laying beside me, she unexpectedly sprang towards our pom-chi, who was on the floor, facing away. The GSD was growling loudly and turning in circles, trying to get to our pom-chi, who was yipping loudly, terrified, and moving away. The whole attack lasted maybe four seconds, as I separated them immediately, but could only do so by physically restraining the GSD. My pom chi was not hurt, but we're concerned that now that the GSD is settled into the home, she will begin to display more aggression. We feel we must return the GSD to the rescue (it's reputable and the dogs are well cared for), as we don't want to risk our smaller dog. Any advice?? Are we doing the right thing? My fiance is devastated and wants to keep her, but we are both concerned for our smaller dog.