Ok so I have this head cannon, that Nightmare Moon was nothing but a demon that took control of Luna and Rarity at some point. This Picture shows Luna still trapped in her own body as Nightmare Moon has taken control. This demon like creature finds somepony that has self doubt and tricks them into letting them take over their body.

Now I am still trying to get my fanfiction that goes with this picture approved, but once it is I will link it in the discerption. I will keep you posted :)