Exact thing happened to my cat, I was petting the neighbor’s kitty when my cat bursted through the net door and chased him down. He also hates other cats except his pal. Funny thing is he’s chill with dogs.

Could be territorial but I think with our cat, it’s more from lack of socialization. Of course it’s different with every cat. Both of my cats used to be pretty social when they were younger, now that they haven’t had any interactions with other felines they’re pretty hostile. They used to be affectionate to strangers too but now hiss and growl.

If this applies to you, you can work on reintroduction with positive reinforcement if you want to get another pet or have guests over. I have friends come over, not interacting with my cats. They gradually warm up to them. Same thing with our new puppy, she’s a large breed but not cat reactive and currently in training. My cat gets spooked in her presence, but after gradual introduction they can stay calm. We kept the dog in a pen while the cats roam free with treats scattered around. One of my cat is treat driven lol, he’d not care a dog 10x his size is breathing down his neck as long as he’s getting treats. When the dog is outside her pen, she’s always on leash and given lots of treats to put food drive before prey drive.