@Background Pony
 
I’m going with the first choice. More detailed.

@Background Pony  
How long did it take you to put that together? O_O

Okay, here’s my idea for this girl’s backstory.
 
Sunset Shimmer was once a unicorn and Princess Celestia’s student, like her toy description says. However, this was from 1,000 years ago. For one reason or another, Sunset felt she wasn’t good enough and ran away. She stumbled upon the Crystal Empire and stepped into the magic mirror that sent her to the “human” world; she also found a magic crown and took it with her because it made her feel better about herself or something. In this new world, Sunset became extremely popular at school and found acceptance as prom queen. However, she let the attention go to her head, and she eventually turned into a mega-bitch.
 
MEANWHILE!
 
As this charming little high school drama went on, the Crystal Empire was being taken over by King Sombra. When he was overthrown by Celestia, who knew not where her former student ran off to and had long since given up on looking for her, King Sombra cursed the empire to vanish for 1,000 years, taking the “human” world with it. Time froze for the world, much like the empire. Then the curse lifted, and time returned to normal like nothing happened. Twilight saved the empire, became a princess, and everyone was happy.
 
Then the movie happens.
 
While going through the history records of the empire, it is found that there is supposed to be a magic crown somewhere that is now missing. Celestia concludes it was taken into the world of the magic mirror, and sends Twilight through to retrieve it. Sunset meets Twilight and finds out she’s Celestia’s new student. She doesn’t like the idea of being upstaged, and now that she has acceptance in this new world, she’ll do anything to keep it.
 
Either that, or she’s just an ordinary nice girl who turned into a bitch and just wants to stay prom queen.

Yep. I see where this is going. Twilight needs to win the title of Prom Queen because the crown that is being given out is actually a magical object of power.

She got eviler as time went on. Being prom queen does that to ya, y’know?

I’m glad they went with the original animation style and not those godawful antro-ish ones.