Having trouble with semi feral and rescue

[Oreosmom]

We live in a rural area and now have 5 neutered cats. We had 4. Two females were born feral but have probably forgotten over the years. One senior male was adopted from a nearby shelter after we learned he had been there 7 years. One four year old male was a stray that we found as a year old cat. The fifth cat was born under a neighbor’s storage shed to a feral female and can be best be described as semi-feral.
Last winter the semi-feral showed up at our house and fought with our young male who had gotten out. The feral was injured and we took him to the vet. While he was being neutered and fixed up, we built an outside well-insulated house for him for the winter. He spent much of his time looking in our windows and apparently decided to be an inside cat. Gradually he let us pet him and we have gotten him in the house many times. He is even litter box trained but the problem is that he and our youngest cat, the young male, still hate each other. We’ve been working on this introduction since last February but they still can’t be trusted together. The semi-feral had to return to the vet last week. He is the one that starts the fights but he always loses! The former stray young male is twice as big but is afraid of him despite always coming out in top of the fight. The other 3 cats stay out of it.
Any advice?

[Cat-FAQ]

The following pages might be useful for you, if you haven't already seen them:

Cat Chat’s Understanding Cat Aggression page, which offers advice on the most common causes of cat aggression, and how to recognise and deal with it: https://www.catchat.org/index.php/cat-b ... aggression

Cat Chat’s Cat Introductions & Hierarchies page, which offers advice on introducing cats and preventing conflicts between cats:
https://www.catchat.org/index.php/new-cat-introductions

You could also email Cat Chat's behavioural adviser for further advice: behaviour (at) catchat.org

Hope that helps.