Perhaps Book Horse might be able to go as someone who will /definitely/ not be there, such as Princess Chrysalis. Alternatively, Book Horse might be able to paint her body and mane white and light blue and go as a spirit. (It may require training your wings beforehand first, since spirits generally don't touch the ground from what I understand.)
You mean Queen Chrysalis. Princess is the traditional title of Equestrian rulers, and she’s not one, no matter how much she’d like to be. It would definitely fit the theme of Nightmare Night, though would be rather hard to pull off. I mean, she has holes in her legs, and it’s not like I can paint on a hole, this isn’t a cartoon.
As for a spirit, you mean like a windigo? That seems rather impractical too. I can’t exactly make myself partially transparent. Well, maybe in the Crystal Empire, but not in Ponyville. Besides, windigos are also more of a Hearth’s Warming Eve concept as well.
Dressing up as Chrysalis was certainly a nice idea though, I would have had that as an option if it was more feasible!