Set this back a 1000 years ago, where three instead of two Princesses ruled over the Earth. Set it when things didn't make sense, and the world still had an evil lurking around. Princess Diane, the third sister, ruled side by side her other sisters, but in faux harmony rather. Daily fights ensure, and one day, Princess Celestia snapped. She sends Princess Diane on one of her mundane missions, which was to ensure the magic binding Discord was reinforced. On the quest to reseal Discord, she discovers the meaning of life (literally), but in return, Discord has been released from Ponydora, a box that had imprisoned him over the last 1000 years. Havoc ensures on Earth, and only she can reseal Discord. Along the journey, she meets a stallion, Cain Tellot, who proves to be stronger than he looks. Will the two undo this evil, or will this friendship of theirs turn aloof? And will the three sisters ever make it up?
Edit: fixed MANY, MANY grammatical mistakes o.o ... and to trust spell check. Next time, manually checking.
Edit 2: Changed description
Edit 3: Changed description yet again... lol... imsofickleminded