Was about to comment something like this :p
longcat is looooooooooooooooooooooooooooong
You use "крупная" when you want to point out how big or fat the cat is. You can not use to indicate that the cat's length and therefore you use "длинная".
I thought it was "Длинная кошка - длинна."
Since the sentence is a predicate phrase, shouldn't the short form be used?
Perhaps. I'm Polish, so I defaulted to how it'd sound in Polish to make a joke. Works 60% of the time, but maybe not here.
Long? Is this normal russian word usage for big animals? Why not болишая?
I'd say it's just about exactly like "a long cat" is in English. It's not common, but it communicates a specific image (pretty well captured by the images above) and means something distinct from "a big cat."
No, it means the exact same thing in russian. It's a joke or an internet meme or something.
Whoa! Those are some REALLY BIG CATS!! Don't leave small children unattended!