The potion used to transform people into ponies has gone through many, many interations. When one of the older potions is used, results aren't quite what was expected.

EDIT: I used a piece of fanart which I thought was wonderful as the cover image :3

Scootaloo is a small pegasus with big dreams, one of those dreams is to be even more famous than her idol, Rainbow Dash. To get there, though, she needs to do a bit of growing, and first of all she needs to learn how to fly. A new invention from her friend Doctor Whooves may get her there in no time at all, literally before she knows it.

Cover image from Snapai! http://snapai.deviantart.com/art/Buck-to-the-Future-260010765