We've made a few attempts to get our cat to co-exist with my brother's cat, which is home for the summer. Both are neutered, ours is male, his is female. They're both rescued off the street and approximately the same age (male is ~9 months old, female ~11 months old).
However, after some attempts and recent behavior, where he reacted with extreme aggression and panic at a cat he saw a couple of buildings away, we're starting to think that his behavior is specific to cats. Around people, he practically never becomes this agitated. He might hide if we bring the cage out and he likes to play rough, but he never seems to feel panic around people, getting comfortable with them relatively quickly.
When he sees my brother's cat however he panics. There's hissing and after a while attempts at aggression. My brother's cat is very calm, she never hisses at him or paws with her claws at him. In fact, last time they met through an opening in the door, enough for paws but no more, she lied down close enough for him to almost be able to claw at her, but when he did she didn't panic, just avoided the clawing. Besides being in a room that he normally can use, she's done nothing to provoke him (and he's friendly with my brother who is in the same room anyway). In the past she barely paid him any mind. They still play under the door though and he isn't aggresive when they do.
Other instances, such as hissing when he sees cats across the street, far away, make us think he might have been chased or attacked as a kitten by older cats, probably ones he felt he could trust and thus built a deep fear and distrust for other cats.
We feel my brother's cat is probably ideal for him to get over this, in time, but I'm not confident this isn't just some sort of male dominance behavior.
Is it normal for cats to be universally aggressive and apprehensive around other cats, even when they are far away and/or non-threatening?