"No Kitty! That's a bad Kitty!"
Mr. Kitty has finally had enough of Cartman's abuse, so she decides to teach the fat ass a lesson. Using her feminine, feline wiles, she has all her adoring male followers, all one thousand of them, attack the new kid until his group surrenders, or dies fighting. Will the boys finally be defeated? Will the cats win? Will Mr. Kitty finally get some of Cartman's Cheesy Poofs?
It's up to the game developers for us to find out!
Since cats are smaller than people, they can be easier to defeat, but maybe their claws can bypass some defensive items or they can have the unique ability to dodge attacks like magic spells, making them harder to kill. Maybe the cats can also have a "cat burglar" ability to steal away items (like potions, etc.) from the players when the cats attack, so it makes it harder to fight them.
That being said, this idea is just mainly posted for laughs, as I doubt the game developers will want to put such an idea in their game. But I do hope any and all readers find some humor in the idea.