A very interesting theory by much of the fandom is the story behind a certain DISCORDED character, that's right it's TRIXIE!
Seriously though, the idea that Discord is (or was) Star Swirl is an overall fairly solid theory based on the fact that is has seemingly extensive evidence to back it up. Here are a few of the facts:
- The first bit of similarity between the two characters is the amount of symbolic little background imagary that is shown in The Return of Harmony Part 1. While being corrupted by Discord, the area behind the main characters is covered with stars that swirl around.
- Discord and Star Swirl have a beard that looks similar (see Twilight Sparkle's Nightmare Night costume).
- Cheerilee told the Cutie Mark Crusaders that Discord "has the head of a pony".
- Discord is frequently portrayed with stars near to him in multiple episodes.
- Discord travelled back in time with Twilight In The Return of Harmony Part 2. Star Swirl invented time travel spells.
For more info, go here: http://mlp.wikia.com/wiki/Forum:Speculation/Discord