New shelter cat brutally hunts down existing cat

[AppleButter]

I have another thread on here about my shelter cat puking up bile within 48 hours of arriving with us. He had severe giardiasis and bacterial pancreatitis and was a cliff’s edge from death. He survived and is an absolute Darling (male, medium sized, skinny, 1.5 years)
BUT whereas he tolerated other cats at the shelter, he now murderously hunts down my existing shelter cat (small, female, 3 years) whenever he manages to slip out, twice. There is no chance to get to know each other, there is no hunting-sussing out-hissing-lurking-leaping , it is pure killing machine. He is bigger and stronger despite being post-recovery.
She is defensive, doesnt attack, just fights him off and then hides and soils herself, and runs away.
I assume he was peaceful with other cats at the shelter because he was weakened and ill, and now, at home, where he feels so comfortable, he is just desperate to destroy her. His attacks are brutal and incessant, and only with great effort can I separate them and help her escape.
I have told the shelter that as much as we adore him, we can’t keep him and he will probably need another home as sole cat. Just wondering whether anyone else ever had this and if there is any magical solution. We have : feliway friends , have been doing scent-exchanges all month, feeding them treats together when theybsee each other through a glass door. Initially I managed to feel them with the door open a tiny slit, but them he broke out and attacked her and she avoids going anywhere near his door. He has a lovely comfy room to himself and when the female cat is securely outdoors he roams the house and snuggles with us. We also spend lots of time with him in the isolated room. He is well fed and is absolutely family friendly, really a dream cat ❤️

He is so cute and loves humans. He just seems to hate my female cat even though she shows no agression to him, just a single hiss at the door, and then she retreats.
I have seen many cat fights with my previous cats, but he seems to instinctive attack her, with no breaks, just a high energy attack. We play with him lots, and he has lots of staircases to run up and down.

I think he is just incompatible with her.

Its not fair on your existing cat to be so frightened she soils herself, he needs to be rehomed.

@KittenCatcher I thought so too, I don’t want to be overreacting, but it has been a month, he knows her from scent and sight, but is still irrepressibly brutal to her.
I won’t send him back to the shelter , I think he would be sad, so I have asked him to be rehomed from here, hopefully he can go from hone to new home, and live happily ever after.
I think it would be evil to take him back to the shelter at Xmas, as he is so human focused and love-hungry.

Nope he needs to go. You've tried everything.

We’ve had the same issue - if an incoming cat is anxious sharing their space then they will try and get the existing cat(s) out. We tried for 4 months but it never resolved and we had to pass the new cat onto a friend who had recently lost a cat and could offer him a fantastic home all to himself.

At the shelter he was tolerant of other cats. Just not here, despite doing most of the slow introduction steps, giving him exercise and the run of the house when the other cat is away. He is very needy, and a real lap cat. I think he would easily find a new home, he is child friendly too, and very beautiful ❤️

Re-home. Not fair on either cat, but particularly your female.

www.mumsnet.com/Talk/the_litter_tray/4379566-If-your-cat-brought-up-yellow-bile-how-did-it-end

It has been a long journey for him, I think he was someone’s lapcat and they dumped him near the shelter because they were bored or because he was ill. Was lovingly cared for by the shelter, I can’t fault them.

Sorry but that is really REALLY unfair on your poor existing cat.

Hmm I'm finding it hard to see how you can enjoy the 'cuteness' when your other poor cat is really suffering. You're clearly a bit blinded by enjoying him being affectionate to you and that's what I think is unfair - you're enjoying it at your other poor cat's expense.

Please send him back to the shelter asap, before your other cat is permanently affected - you could easily end up with no lovely affectionate cats at all at this rate. She will be under huge stress. How about a few heart emojis for her instead?!

I'm also struggling a bit to understand how you can talk about him as an "absolute darling" and yet also say he's "out to destroy" your other cat.

Feeling really sorry for your existing cat.

You do seem very attached to him, given the choice which one would you rehome. Your female cat must be very miserable and deserves to be in a safe loving environment. It may be difficult to rehome him directly atm, leaving things as they are js not really fair.

No worries, he is locked away, isolated in a separate room. I am now very careful that he doesn’t get out again, at feeding time I secure the girl cat in whatever room she is is in, before going into his room to feed him and clean up. Just in case he slips out again.
They are not together, just two occasions when he slipped out.
I have already notified the shelter that he has to be rehomed, I was just wondering if I was being too anxious. Obvs am not, and it does good to hear it from a third party, i needed to hear this. Thank you.
I promise he is in a secure and comfortable room, and girl-cat is safe ❤️

Honestly OP, your responsibility is to your existing cat. You may be doing your very best to keep then separated, but even if you 100% succeed, your original cat still knows he's there and is living in fear of him.

You can't keep him - he needs to go back.

If you rehome him from your existing home rather than returning him to the shelter now, there’s a chance he’ll take longer to rehome because people will have to make a separate journey to see him at your house. And your female cat will still be able to smell him and will remain distressed.

Please return him to the shelter and put your existing cat first.

You did say he has plenty of stairs to run up and down and has run of rhe house when your female cat is outdoors. They will still be able to smell each others scent and are probably both very stressed. He will not enjoy being confined to one room especially if his food and tray are in there and she wont enjoy being terrorised or feeling anxiius.

I can see it’s not a decision you have taken lightly and have tried to make it work. Don’t be hard on yourself you have done your best and continue too, as some would have just dumped him back at the shelter. As you say he needs a home as a sole cat, and I am sure he will be just fine, the trauma it’s causing your other cat is just not manageable as it can cause so many health concerns. I have two cats one hates the other but it’s manageable so I do understand to a certain extent.

Of course he can still be a darling and do this. He’s not attacking because he’s a horrible cat, it’s instinct and his previous (lack of) socialisation.

OP, I think you’re sadly doing the right thing by rehoming him. It’s not fair on her to be attacked and it sounds like it’s worse than the usual fisticuffs that are normal when introducing a new cat to the home. The only alternative is to keep them completely separate at all times, which isn’t practical for most people.

Thanks everyone. I have accelerated things by sending photos to the shelter and asking if he can be put on their website. Give him the best chance of finding a great forever home. He will make a fab pet to someone. Will speak with them tomorrow to discuss plans.
Yes, he is being a thug to existing girl cat, but doncats have moral agency in that sense? I still have a duty of care to him, since I invited him into my home. I have to feed and look after him, make sure he is emotionally well, and make efforts to make it work. Now after the 2nd attack , despite all efforts, I have a strong feeling that this instinctual drive isn’t a one off and can’t be coaxed out of him. So now to find the kindest way forward. And girl cat has priority and will be kept safe.
I know cats don’t realise it is Xmas , but we do, and there is something morally reprehensible about dumping a cat back at the shelter just before Xmas. The cat god is watching and wouldn’t be happy.

I suspect a female cat will always be at risk of this behaviour if you take on another male cat - somehow lone female cats always seem to be targeted by other cats.