I'm not sure if this is like Disney Princesses as Monsters or anything but I can see Rapunzel as a mummy (her hair being what covers her) and Ariel as the thing from the black lagoon or some kind of sea monster. Tiana could be a swamp monster and umm perhaps Aurora as the bride of Frankenstein.
I think this is my personal favorite. She is very cute! Also, if you wanted to stick with the main "Princess" franchise you could also, do Mulan as a dragon and Pocahontas (I can't think of anything for her)