Princess Luna (A.K.A. Best Pony Evah)
Ah, the princess of the night. An Alicorn far more regal, beautiful, and majestic than Celestia could ever dream to be. Sure, she turned evil, but it was only because Celestia was described as all powerful and beautiful and perfect, and her sun was loved, appreciated, and eagerly anticipated by the ponies (Examples: Fun in the Sun, beautiful light of day...And how come the sun gets a smiley face and blush in kids' books?). Overjoyed Celestia gladly soaked up all the attention and was never there for Luna, brushing off her plights by saying stuff like "Don't worry Luna. I'm certain they love your night. Just give them some time to come around".
To the ponies, the night was a dark, fearful time meant only for sleeping through (Seriously, Wikipedia said that like in folklore or something the night is evil because there's shadows instead of a radiant sun and criminals run around and...*Sigh*), without the bright light of the beautiful sun shining across all of Equestria and bringing life to plants, animals, and ponies everywhere. Luna even raises each and every one of the stars. That must take a LOT of magic, and no one appreciated it. Celestia and all of her subjects forgot all about Luna. There's no telling how long she suffered before she reached her breaking point, and let her dark side conquer her. She imprisoned Celestia in the sun (Watch season 1, episode 2 again, if you will. It really looks like the sun starts to glow and then Celestia appears out of nowhere like she had just escaped from being in the sun a second time), and cast Equestria into eternal darkness. Celestia unexpectedly escaped with her powerful magic and was somehow able to utilize enough power from the Elements (She was only conncected to three of them, and Luna was connected to three of them. Lauren Faust said that in the Elements of Harmony official guidebook. But that doesn't make sense. How can Celestia defeat Luna with three Elements, but the Mane 6 can't defeat Discord with 5 elements?) to defeat her corrupted sister and banish her to the moon for a long time. I wonder how she was trapped...Maybe she was turned into the purple cloud of starry mist (which she transforms into many times in the first and second episodes) against her will, pushed all the way to the moon by a sonic blast of harmony and friendship, and when she reached the surface she was magically sealed inside the moon as the mist stuff, appearing as craters to the ponies below. I don't know... :-\ :P
Then after 1,000 long, cold, boring years (:P) the spell was broken and the evil Nightmare Moon somehow knew exactly where to go to reintroduce herself and tell everyone her evil plot (Anyone else ever thought of that?). Then she went to Canterlot Castle and imprisoned Celestia in the sun, perhaps after a short chat, then went back to Ponyville and saw Twilight Sparkle and her soon-to-be friends reading about the dreaded Elements of Harmony. She tried to stop them multiple times on their quest, but failed and was forced to face Twilight personally. I wonder what she was planning to do when she was charging at Twilight. Her horn wasn't glowing but her head was lowered... *Gulp*
When the Mane 5 showed up, Twilight realized that what she had denied and not cared about before was true: She had true friends, and she cared about them very much. She felt the spark of true friendship in her heart, and the sixth Element appeared: The Magic of Friendship.
The evil in Luna's heart was then purged by the rainbow made out of the magic of friendship, and Princess Luna was reborn, but with very low magic levels. It never showed her in between Friendship is Magic part 2 and Luna Eclipsed, but either that was one of Hasbro's schemes because the little girls didn't like Luna, or Faust deliberately did that because Luna wasn't used to being around other ponies after 1,000 years of nothingness, and she was still regaining her good powers she had before she turned into Nightmare Moon at all. Then when she got them back she tried to make a good appearance in Ponyville, but things didn't work out, due to her flair for the dramatic and her use of the traditional, royal Canterlot voice that she must of thought was still in full force ("It is traditon, to speak using the royal "we" and to use THIS MUCH VOLUME when adressing our subjects!"). Wouldn't it be weird if they made a movie (Or possibly episode, or maybe even a whole season...!) about their past and Celestia was using the Canterlot voice? :D
Thankfully, Twilight was able to help the night princess and let her know more about how modern Equestria worked, and eventually, with some...difficulties along the way, Luna even agreed to take the form of her conquered dark side just to play along with the silly celebration she had once before tried to turn into a "bright" and "glorious" feast.
Luna realized that things had changed over her 1,000 year banishment. The ponies no longer ignored the night, and they loved her just as much as Celestia. :)
"OH MOST WONDERFUL OF-I mean, oh most wonderful of nights."
TO BE CONTINUED...MAYBE.