As you can imagine (or i did): Edit
she's Pinkie's doppelgänger if you don't know what that is here's a link for more help: http://en.wikipedia.org/wiki/Doppelgänger So now that you know what it is why is Surprise here? well she's kinda Pinkie's twin sister (that's what she call's her) but one time Pinkie found a chamber in Celestia's castle, and there was a mirror (you know from the up coming movie as an example) but then there was another one, it wasn't like the one in the movie that has stairs or what ever, it hangs on a lower part of the wall, when Pinkie poked her head in it, the reflection moved like water and in it was a 'Parallel ponyville, all the ponies like you know look like there original's. (see 1st video below for example) 'Check out this Gallery and find Pinkie and her to find out what they did in that crazy world. http://mlpfanart.wikia.com/wiki/Surprise/Gallery'
the vid show's an old G4 episode, just like the girls they had the same adventure's but at a different time but same places but different looks. What's weird is that when i watched the old stuff for the first time on Youtube (not first movie or episode) for the first time it showed that she had an Alicorn look when she was flying.