TL;DR If anyone else could give me a character list, that would be wonderful, thanks.
I try not to complain about this... Sometimes I just don't understand the art, or in this case, I don't know enough about it. So I'll open up the picture for the artist's description. I really like the picture, but I need a little more context to fully appreciate it. I'm assuming that each of them are dressed as another character that their JP VA's are known for, but I don't know who the characters are.
I posted this a little further down, but I'll repost it here for you:
Rarity, AJ, Dash, and Pinkie are played by Mikoi Sasaki, Sora Tokui, Izumi Kitta, and Suzuko Mimori respectively. Those four voice actresses also play the four leading characters of an anime called Tantei Opera Milky Holmes (a detective-themed comedy show), and that's who their four pony roles are dressed up as (though Rarity and RD have switched outfits here).
Twilight sparkle is played by Miyuki Sawashiro. In an anime called Durarara!! (which I don't know as much about as I would like to, sadly), Miyuki played a character named Celty Sturluson, who Twilight is dressed as here.
Finally, Fluttershy is voiced by Emiri Katou. In Mahou Shoujo Madoka Magica (A freaking AMAZING magical girl anime (seriously, it's completely awesome)), Emiri played a magical creature named Kyubey who assisted the main characters (assisted... yeah, that's a good word), and that's who fluttershy is dressed as here.