It involves Fluttershy eating a poison joke cupcake (who ever could have made that ^_- ) and transforming into an anthro pony version of Jessica Rabbit.
Lots of possible names for her here. Jessica Pony, Hootershy (thanks for that) but we're going to go with Jessica Shy ^_^
Enjoy!
(Note: If you find the easter egg, the music is from Mass Effect 2 )