That happened to my five year old cat Dudley. When we brought Sammy home, Dudley hissed, wacked, and growled at Sammy. He ignored all of us for several days. It turns out Dudley lost 3 pounds from all of the stress.

Now they get along fine, and we are working on getting Dudley's weight back up. He is currently on a specail diet from the vet to re-build his weight.

You can see a picture of them both in my signature.