I lived with 2 other roommates for a year and decided the rescue a dog. She was a 1 year old German Pinscher. She weighs 45lbs. She has always had issues with cats, she has never attacked one, but she will chase them if she can and the times she has caught them, or the time I visited a friend's house who had a cat she keeps them cornered and just stalks them. She doesn't lunge or try to bite them, simply stalks them and mirrors their every move keeping them cornered. After 1 year, I moved into a new house, the people I'm living with have 3 dogs of their own, small white dogs. My girlfriend also got a new dog last week and they got along great until the new dog would try and take a toy that my dog was playing with. They would play with the toy, my dog would chase my girlfriends dog, but when my dog finally got the toy and my girlfriends dog started chasing her she would growl and sometimes it even turns into a fight. Now, sometimes when she is running around in the backyard with my girlfriends dog they'll be playing and all of a sudden it turns into a fight, you can see it escalating sometimes and we stop it before it turns into a fight but sometimes it escalates so rapidly we can't stop it before they start fighting. Also, with the new dogs she stalks the smallest dog similar to the way she stalks cats and if the other dog is sitting on a couch with a human she'll whine while staring at the dog. Lately she's even nipped at this small dog, it's happened twice now. Whenever I see her doing it I tell her immediately to leave it. She'll leave the dog alone for a little while, then in 1-2 minutes she's back to stalking her. Any help you guys could give me would be greatly appreciated, it's really embarrassing around the new people I'm living with when my dog stalks and whines at their dog.