What to do about a neighbors dog that keeps attacking mine?

The neighbors dog is a german shepard mix. It is not very well taken care of it does not get fed on a regular basis. I have a neo mastiff very laid back dog. The german shepard came into my backyard today and started atttacking my neo. My neo got hurt pretty badly the first go round. Well as soon as i let my neo out to use the bathroom here came the german shepard again and was attacking. I dont know what to do about it and was also wondering why the german shepard would be doing that? Now I cant even put a collar on my neo because it hurts him to bad because most of his wounds are all around his neck.