So, after weeks of speculation, we finally get a synopsis. Let me read this off:

"When Starlight Glimmer starts becoming friends with a fellow formerly-bad pony, Twilight tries to stop her."

Wait...who's this "fellow formerly-bad pony"? I have my thoughts, but I want to know yours, so please tell me in the comments below.

Anyways, my guess will point to Trixie.

She's been the "The Great and Powerful Trixie" for most of her life. From what I can tell, she acts like a jerk, and punish anyone who disagrees with her. For example: Pinkie Pie's mouth was psychically removed. I don't understand how she pulls that off.

At the end of Magic Duel, Twilight tricks Trixie think that she can pull out an aging spell, a "mare to stallion" spell, and create duplicates of Rainbow Dash (Pinkie Pie being forced to play 10 instruments does not count, because she's just being Pinkie Pie). After all of this, Trixie learns her mistakes, and "The Great and Apologetic Trixie" was born.

This might be my guess, but what's your guess on this "pony"?