Yes this is Nyx. In "Past Sins"'s sequel "Winter Bells" Twilight developed for her improved disgusie spell which was casted on the headband. The spell changed coat's color to pure white removed wings and changed dragon eyes to normal pony eyes. Totally not a Nightmare Moon.
The picture is experimental in nature while green headband is canon for filly Nyx she is not often drawn with it as adult. So I wanted to test how this will look.
So about the swimsuit. When I've bought "Them's Fighting Herds" game https://store.steampowered.com/app/574980/ I noticed that some of Oleander's color schemes looked like one-peice swimsuite and it was fanny. I've gygabytes of pony art but I never saw anybody tried something like that on normal feral pony. So I wanted to fix this. Just for LOLs and more totally absurd but sexy clothes on pony. :)
BTW I don't know should I use tags like "Winged unicorn" or "alicorn". X) The wings would reappear imediately if she takes the headband off.
