You didn’t mention how old the bully is, but if he’s (I’m assuming it’s a male) between 2 and 4, chances are he’s entered “social maturity,” and that’s when they become even more territorial. I had the same thing happen with one of mine. He was a wuss when he arrived around age 2 or 3 and loved being part of the gang (there were five then, four now), but after about 6 months here, he started tormenting one of the others, who is now terrified of him and if I don’t keep watch, will pee outside the litter box. (The bully will ambush her if I don’t separate them when she needs to pee and poop.)
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