Get sachets of lick e lix or whatever tour cat perceives to be 'cat crack'.
Put in same room supervised. Immediately dish out the salmon lick e lix in a bowl or large plate. Wave it round so they can smell it. I united mine through making them eat together and lots of praise. Then remove one of the cats again. Keep doing short and positive introductions. Hope it helps.
The same thing happened with our cats. They are brothers and house cats. They are extremely close and one of them got out once and was sprayed by a stray. When he came back inside his brother didn't recognise them and tried to attack him. It went on like this for a couple of weeks. We tried bathing them so they'd smell the same, feliway and feeding them treats together. Nothing worked. Eventually we borrowed a puppy cage from someone and shut them in the spare room. We would put one in the cage with food and water for the day and the other would be outside the cage but locked in the same room. They grumbled and hissed but eventually slept. We did this for a week alternating which cat went in the cage. After a week they were tolerating each other and then next day they were back to normal and are now closer than ever. It was very stressful! I hope you get it sorted soon.
PS - sorry, there is one other possible explanation which is that he's in absolutely tremendous pain or discomfort from something. I know you said it was just a few steps he took but could he have cut or injured himself in some way ? Cast your memory back.
I really think I'd call the vet so that she can check him out. (It sounds as if you won't be able to get near him to investigate.)
I can only guess some trauma trigger. It must have been one hell of a trauma though.
I'd keep them in the separate rooms (or rather him - she can be allowed free rein of the house because she's normal and to be locked up might agitate her unnecessarily.)
I'm not normally one to recommend sedation but there's some milk based sedative - I think it's called zylkene - that has been recommended on the board before, including by some of the vets posting. Maybe your vet can advise. I think I'd phone the practice and ask for advice.
Completely stumped here too. Do you know what his background was before you took him in? I wonder if going outside triggered a traumatic memory for him? In which case perhaps feliway would calm him back down - nothing to lose from trying it, anyway.
The only other thing I could suggest would be going back through the introductions process and basically keeping them separate as much as possible then slowly bringing them back together under controlled conditions (i.e. lots of treats...)
We have 2 cats. Our old girl is 14 and we have had her since she was a kitten. Shes a real homebody, mostly stays in the house but if nice, will occasionally sit on the patio in the sun. Sadly we lost her brother a couple of years ago. Six months ago we took in a rescue, hes about 15 months old now. Hes never been out - hes not interested in going out and usually ignores an open door. Although he was quite nervous when he came, hes settled in well.
They have both been getting on great and will play and cuddle up and sleep together. However our old lady went out for a short time today, its the first really warm day we have had this year - she usually just goes out in spring and summer so he has never seen her go out before. The back door was open and our rescue boy went out to see where she had gone. He dashed back in after just taking a few steps. she followed him in. Its now utter hell.
He doesn't seem to recognise her. They have been fighting constantly since. She growls at him, his tail is up like a loo brush and hes making strange howling noises. He wanders round the house as if he doesn't recognise it or us - hes sniffing everything and very scared and nervous. He will go to DH but he won't settle with him. Hes turned on me and sliced my hand open. Hes also turned on dd1. Theres been a few huge cat fights this evening and we now have them in separate rooms. Our old lady is on heart medication and I don't want her to have the stress of this as I'm worried it will finish her off (although she is quite capable of beating the crap out of him and has done today - several times).
Has anyone any idea what the hell is going on? I've been owned by cats for years but have never come across anything like this before. Everything has changed in an instant - before she wandered out they were on our bed together. Has anyone any ideas as to how we can get things back to normal? Would feliway help here?