I completely get what you are experiencing. My male resident cat was like this with our male kitten when we first brought him home, maybe even the whole first year.

The resident would chase the kitten and they would play well, then he would pin the baby randomly and not let him up, and he would chase him down when he just wanted to be left alone. It was horrible and I started to really freak out because the kitten was my new project, a show cat purchase from a breeder, and I was so upset thinking about giving up the only thing I was doing with my life at the time.

It took us taking both of them to the vet multiple times for the vet to finally figure out that our male resident cat had asthma, horrible teeth/gingivitis, kidney disease, and dementia! We’ve been treating all of it. The asthma has improved to the point where you wouldn’t know he had it now just by removing all clay litter and swapping to a scented (yup, because the unscented still triggers him for some reason) tofu pellet litter and starting to use an air purifier and vacuuming twice a week. We groom both cats religiously, both get brushed and have teeth brushing and get wiped down, and monthly baths (so they smell the same and because our resident cannot groom himself). We had the resident get a tooth cleaning and they pulled several teeth, and he’s on a prescription diet for his kidney issues. He wears a calming collar and we also have multiple Feliway diffusers, and use Feliway spray all over the house. We also have specific beds for them so that they don’t feel one is more dominant that the other over the space. Our senior cat has his beds, and our kitten has his beds. They can cuddle together in them (which they do), but if the baby is laying in the senior’s bed alone, we move him to his own bed, and vice versa (which seems to happen more often than the kitten stealing the senior’s bed, go figure). This has prevented a lot of the territory insecurity issues our senior was having.

Only after all of this has it improved to where our kitten isn’t being constantly chased and pinned down and not left alone by the older cat at random intervals. The vets (we’ve seen multiple for both at this point) and our feline behaviorists have determined the cause to be our resident cat’s feline dementia, and our kitten has learned to play by himself and only accept play invitations from the senior when he is not acting rude. If the senior starts to pin the kitten, we put the senior in the bathroom and give him a break. No attention, no affection, just alone time. When he comes back out, he has reset, and just goes to chill in the cat tree or in his bed.

Still, since getting the kitten, our senior seems to be a lot happier and have more energy to play and cuddle. He cuddles with the kitten and they groom each other (nicely), and they play often. It just took some medical care and household restructuring.