I adopted a senior (him, 8) at the end of 2025 and in February “got him” a kitten (her). Things were going so well until recently, he has been hissing at the kitten. She’s not even within whacking distance when this happens. One day my dog walked over him, and I’m certain she did not step on him, but he hissed at her and swatted, too. I noted his eyes seem more dilated (though not when this is happening) and when mentioned all of this to the vet so we went in to ensure blood pressure wasn’t elevated/causing vision issues. I also had rearranged and been trying to change his food during this time. I’ve put the furniture back and stopped trying to transition foods. The only thing environmentally that I haven’t done is put his feeding spot back where it was before this. I also noted recently that he (senior) has been resting with his mouth slightly ajar. Which I just puzzled together and haven’t notified the vet, but planning to.

Is this something “normal” that can happen as a kitten ages, other cats don’t like them?? Does this (hissing seemingly out of nowhere) ring bells for anyone as a behavior their cat has had, with or without a kitten involved? What did it wind up being? Thoughts on what to ask the vet to check for? He had a pretty thorough physical exam when we had the blood pressure checked and nothing seemed off. I’ve only had him 4.5 months so I’m still getting to know him, but something like this is rather out of character so I’m stumped on what the issue might be.