New cat from outside, nicest thing ever. But my other cats don't think the same.

[PhysicalSadness]

So I found a stray cat from outside, he is the nicest thing, he showed up in my backyard. We checked for an owner, got him scanned for a microchip, nothing. He is ours now. So we finally let him out of the basement to roam the house, he is super nice and friendly, when he met our other cats, he obviously meant no harm, he wasn't hissing or showing any signs of aggression towards them, yet they still hissed and other things, they ran and hid from him. He was just trying to be friends with them though? How do I get them to warm up to him? Any tips?

Thank you.

 

[gilmargl]

If they are only hissing and hiding - that is normal. It may take a week before things quieten down. I find it easier for all concerned if I take the new cat back into his room or back into the basement at night (with a few treats). It gives all the cats the opportunity to act normally for a few hours and for you to get some sleep without being woken up by cats hissing under your bed.
You mention "other things". Serious "other things" are real fights, with flattened ears and screaming. Non-serious "other things" are puffed up tails, growling and some lashing out with paws when a cat comes too close. Since fostering strays, I have successfully introduced many female foster cats and young kittens to my basic 4 (all female). A recently spayed older male was simply no-go. His presence in the house in his own room frightened them all and they would sit staring at his door, too scared to move. Letting him out was not a good idea but fortunately he soon found a new home.

I have never made a big deal out of introductions. If you are nervous, it won't work anyway. But it sounds as though things are going fairly quietly. Pay more attention to your own cats than to the newcomer. If you are worried, you can take things more slowly. There are many examples of how to do a planned introduction on this site. Perhaps I have just been lucky.

 

[Maurey]

Did you start doing slow introductions while he was still in the basement, before you let him out? You need to slow into him into the household if you want the cats to get along. Plenty of resources on the topic.

How To Fix An Unsuccessful Cat Introduction - TheCatSite

Unsuccessful cat introduction can lead to stress and conflicts in your furry family. Discover the steps for a proper reintroduction.

thecatsite.com

How to introduce new cats to your home

Bringing a new cat home is a big deal for your resident cat(s). Here's how to introduce cats.

www.humanesociety.org

Introducing another Adult Cat or Kitten to your Cat | International Cat Care

icatcare.org

You need to go at the pace of the slowest cat or cats, which, in this case, will be your residents.

 

[vince]

Probably the first thing is to make him smell like he belongs, cats being very smell-oriented. Scent swap by rubbing the new cat with a cloth, then rub the others with the same cloth, then rub the new guy again with the cloth. Do this daily for a few days before more formal introductions. Also, if you can isolate the cats into groups, you can let them roam about in the others' areas. Others have tried putting a spot of vanilla extract behind the ear of all the cats, with good results. I've not had to resort to that, so can't speak to its effectiveness. Hey, it's worth a try!