Sorry this is so long, but I’m looking for advice…and I just need to vent at the same time.
About a year ago, we were adopted by a VERY emaciated adult male cat. The vet thought he was probably on his way to the woods to die when he ran across our backyard and I spotted him. We nursed him back to health, quarantined him on our screened-in porch for a few weeks with the guidance of our vet, and gradually began the process of introducing him to our household. We already had two male cats that my husband rescued when they were kittens in 2005. The introduction of the new guy into the house was pretty uneventful after we gradually let the new guy out of his bedroom to hang with the other cats. We took our time introducing them slowly, and kept the new guy locked in the room when we weren’t around for quite awhile until we were comfortable they would be ok. The three of them get along well enough, although the new kitty is definitely trying to be the dominant one and sometimes would start scuffles. For the most part, all was well.
Fast forward to last Thursday night. We’re getting ready to close the house up for the night and realize that we had left the back screen door open later than we normally do when we hear our cats howling at something outside (ours are strictly indoor cats). As we go into the kitchen to shut the door, the two of our cats sitting on the stools, growling at whatever was in their backyard, suddenly turn on each other and all hell breaks loose. It was unlike any other cat fight I’ve ever witnessed in my life, and I’ve had cats since I was born.
We separated everyone into separate rooms, thinking they just needed to chill out. Try to let them out a few hours later and all hell broke loose once again with the two. Separated them for the night and went to bed, as it was almost 1 a.m.
The next night, we tried to let them out again. Same results. Apparently our big happy family isn’t so happy anymore.
After doing some research online and talking to our vet, it seems as though the cats have displaced aggression toward one another, and everything we read or were told is that we have to reintroduce the cats to each other as if they are brand new.
For a week now, we’ve had the two “original” cats together and separated the newer guy, as he is the aggressor. We’ve split the time that they are locked up in a room and when they have free rein of the house. We’ve been feeding them on opposite sides of a closed, solid door, and my husband built a screen “door” so we can start to introduce them again.
Today, we put up the screen and covered most of it in newspaper, leaving about a two inch gap where the cats can see each other, which is what our vet suggested. We left the aggressor in the room behind the screen and let the other two out of the bedroom they were locked in. The other cat involved in the original scuffle comes to investigate and is totally fine. Tail up, curious, and no noise. Aggressor cat is ok at first, then hisses, then starts the low growl “I’m going to kill you” rumble with the tail starting to get big. We shut the solid door and left it to try later.
I am so frustrated right now and don’t know what more to do. Has anyone dealt with this before? I am looking for any advice or wise words that you may have. The thought of having to either keep them forever separated, put the aggressor on kitty prozac, or get rid of the aggressor kitty all make me well up with tears. 
Any thoughts or advice would be greatly appreciated, especially if you’ve dealt with this before.
He was obviously someone’s pet that either escaped and lost his way or was abandoned.
I hope there is smooth sailing from here on.