I don't know if you'd include the frowning/daydream critters in your au, but if you do, here's some ideas if you don't mind
Catfeine - Slendercat (just imagine pitch black skirt suit, paper-white fur, and her sole facial feature is a permanent toothy grin)
Dogpressed - Wendigo (who laments her condition)
Groddy - Oni (hulking, club-toting demon)
Icky Piggy - Zombie soldier (inspired by the Attack of the Dead Men in WWI and the Sabaton song, picture him welding a rifle that never runs out of ammo and emitting toxic gas most of the time)
Floppy - Nuno (Philippine goblin who curses anyone who disturbs their homes with mutation, disease, or just straight up bad luck)
Unique - Each-uisge (pronounced ack-ish-kah, it's basically a aquatic horse that, the moment someone touches one, they become super glued to it, are dragged into the nearest body of water, and become unstuck only to be torn apart)
Sorry for the infodumps, kind went overboard.
I would LOVE to do this. But I don't know if the creator of the Frowning Critters would be okay with me changing them like this. Either way, I love the ideas. THANK YOU SO MUCH.
(And the Nightmare Critters are already on their way. Please bear with me.)