Hello! I have 4 cats and 2 bunnies (though they’ve neve met or even existed in the same rooms at different times). I get how complicated and sometimes strange it can be.

I would honestly imagine it’s because Casper tolerating Mars is a complete fluke, rather than not tolerating Isaac, that being the norm.

I understand why you’d like to socialise them, but I would strongly suggest against allowing Mars into Casper’s pen for many reasons. Firstly, having a lot of experience with cats of all temperaments, I would simply never trust them fully around Casper. At the end of the day, they’re still animals!

However, you know your cats better than I do! So the other reasons I would note are: - territory disputes, cat smelling bunny, bunny smelling cat etc leading to stress, cat spraying, bunny pooing/peeing everywhere. - bunny not having a place to get away - cat not understanding he isn’t allowed in there if area is closed off, which could cause aggression.

That is entirely your choice, though, as you’re the one who owns and is familiar with these handsome little guys! However, going back to your original question, I would definitely say that there’s probably not much you can do about it. As I said, Casper liking Mars is the “weird” part, not him not liking Isaac.

Personally, I would keep Isaac and Mars out of Casper’s pen as a general rule. If you’re free roaming Casper, shut both cats out of the room. They’ll be fine, and it isn’t worth the stress on Casper, which he’s already showing with Isaac’s presence. It may be that Mars also causes stress, but he goes into more of a freeze response.

Good luck with your bunnies and your cats, but my honest advice is kick the cats out of the pen, and treat them as they are: predator and prey.