Starswirl the Bearde/Discord, Celestia, Clover the Clever or an OC.
I'm one of the conspiracy theorist bronies, so I love following people who analyze the plot threads and the fandom and putting my own crazy theories together. That being said, I watched
this video this morning. And while I won't be adding "Discord is Starswirl" to my headcannon, I wouldn't mind RPing that story!
Plot: Starswirl the Bearded is a rather solitary unicorn who is studying magic, his only close acquaintances being his on again/off again lover, Princess Celestia, and his apprentice Clover the Clever. He is often frustrated by the alicorn, however, in her constant nagging of him to make friends while he'd rather focus his time and energy in discovering the most powerful kind of magic. He starts crafting a spell that is supposed to improve his magic by first scrambling it. However he doesn't know how to finish the spell, so he casts the first half and turns himself into the universe's first and only draconequus.
Now Starswirl's magic is different alright. Now, his magic seems to run by the rule of insanity. He casts a simple teleport spell and turns his favorite chair into ice cream. For a unicorn who had dedicated his entire life to studying and improving his magic, this is indeed a setback. Yours is a character whom he is willing to share his state with and may try to help him. But ultimately, I want this to play out as a person with a disease. Only Starswirl is powerful enough to reverse the spell, but the only way to master his new magic is to sacrifice his insanity, so I want a tragic ending. Fun, huh?