About 2 weeks ago, my father and I, who own a 5 year old Korat neutered Male moved into his girlfriends house, and she has a cat, american shorthair neutered male who is much smaller than our cat. Our cat, Shadow(The Korat) is generally pretty nice, and doesn't get into fights, but with the girlfriend's cat, Jack, he is always attacking him. Jack barely comes inside now because he's afraid of Shadow, even when he's locked in my room.