Why does my new dog try to bite my dogs neck?

well, my new adopted dog every time i play with both of my dogs fetch my new dog just runs towards my female dog and trys to get her neck of will chase her down. my dog is 1yr old chorkie nd my new adopted dog is 1yr old terrier mix why does he act like that i say no when he trys to do it he stops but when i throw the ball again he goes after her again like always how can i make him stop this is kinda bothering me.