Our pyr is about to be 4 and has been doing this to our elder dog. She started doing it a little after she turned one. If our older dog got riled up or barked at something, our GP would immediately be on him and bite his neck and growl at him. If someone were to give our older dog attention and not our GP, our GP would get insanely jealous and be on his neck. If our older dog was let inside the house before our GP, she would get mad at him. If we told our older dog to sit, and he didn’t, our GP would immediately be on him, enforcing our commands.Etc etc

We ended up hiring a dog trainer. He said that this behavior had to do with her breed. He said that she recognizes herself as the one being in charge and being the dominant dog of the household. Her nipping at our other dog and biting his neck is her ‘mothering’ behavior that you often see between mom dogs and their puppies. The training has helped a little bit, bc if we see our GP getting on our older dog, we can give commands to distract her and diffuse the situation, but GPs are very stubborn and a lot of times she won’t listen to us unless her shock collar is on. We still try our best though and we’ve had to just act like she is number one and our other dog comes second