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 >>/1654/
http://mlp.wikia.com/wiki/Tempest_Shadow

> she actually looks that way in the movie and isn't some kind of gothy shy and character who they only added as a side hero character for celeb voice reasons.

nope ,she is not a hero,but the main villain voiced by Emily Blunt and she appears during the whole movie. I am posting the screencaps (unedited) and the wiki to show you that I am not making anything up nor I invent her image to make her cooler. In fact, I have posted her IDW comic in the other thread to give proof of her existence beyond the movie. 

> The bats give it away as a Halloween episode, so they don't count.
the funny thing is that neither Fluttershy nor Inky Rose are getting dressed up like that because of a Halloween episode. Inky Rose appears in Honest Apple (Season 7 Episode 9) for judging her clothes style unrelated to Halloween. 
Fluttershy in Fake It Until You Make It(Season 8 Episode 4), while she has been and acted like a bat in a Halloween episode, she has that make up in order to convince a customer. 

Nope, the Halloween episodes you are looking for are Luna Eclipsed (S2E4) and Scare Master(S5E21). Those are about the theme you are trying to guess with your post.