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[Stargazer]

My guess is it could be a cyst or even the beginning of an abcess from a cat fight. I didn't see where it was on his back but if near the neck, could also be an irritation at an injection site. I would be very careful letting him around your cat just in case he has FIV or FeLV (leukemia) or something else contagious (it would be good to have him tested for both). Beautiful cat, though. If the lump gets bigger over the next few days it would probably be an abcess and will require veterinary care.

I had a similar thing happen once. My boyfriend and I lived in a townhome with our 3 cats. One of them, Daisy the Demoncat, was the terror of the neighborhood. She beat up all the dogs and even chased the other 2 cats around. One day she came home with a black cat called Knot-head (name on tag). Daisy and Knot-head were the best of friends and she would invite him over all the time. Then she would pretend like she didn't know us, you know, like a teenager would. It was pretty hilarious!!!