What do you do when your pets do not like each other?

There is most always a period of adjustment when introducing a new cat to a household, with differing levels of tolerance depending on the cat. When I took a feral adult into the house, the two of the three cats that I already had, who were about the same age, hated her and continued to dislike her for the next fifteen years, but they quit trying to hurt one another. Their mother didn't mind the new cat at all, although I can't say they were fast friends.

It turned out that she was pregnant when we took her in. She had four kittens, three of whom we found homes for, and every cat in the household adopted her kittens as their own. As an adult, the one that we kept often sided with the other cats against her mother in disputes.

As with people though, there is only so much that you can do. You can't make one cat like another; the best you can do is to keep them from hurting one another until they've had a chance to work things out.

That means the slow introduction that others have mentioned here, which we weren't able to do with the feral (Bird), as we were in the process of moving at the time that we took her in.

They did learn not to hurt one another, and Bird seemed to enjoy annoying the other cats as much as they persisted in letting her know that they didn't like her, and there were times when I caught them huddled together. In fact, I would sometimes keep the temperatures down at night to encourage that.

For the most part, I think it's best to let cats work out the cat things.