Yes, the black and white pegasus who steps out of the glowing mirror seems very nice, but Pinkie's twitches tell her that this is her evil twin. And when Pinkie makes her own journey through the mirror to meet even more Pinkies and even more Pies, well, that just means more fun! Doesn't it?
I've been haphazardly planning this since March 12, 2011, the day my story "Pinkie Pie's Evil Twin" went up on Equestria Daily. A year and a half later, a little rewriting has turned it into the first chapter here, and the rest will follow in due course. Twelve chapters, maybe? I guess we'll all find out together!