Discord (voiced by John de Lancie) is the spirit of chaos and disharmony who once ruled Equestria with chaotic power until he was turned into stone by Princess Celestia and Princess Luna with the Elements of Harmony. He returned many years later, but was later defeated by Twilight Sparkle and her friends.
Sometime later, he was brought back under the request of Princess Celestia who wanted to have him reformed. Fluttershy was the only one willing to do this, and at the end, she succeed in changing his way. From then on, Discord became an anti-heroic ally.
(Spoiler Alert: His path of redemption came full circle when a evil magic stealing villain named Tirek arrived. He convinces Discord to go back to his old ways and later betrayed by him. Discord helped Twilight and her friends defeat Tirek by giving her a necklace Tirek gave him earlier. This necklace turned into a key which was needed to unlock to power to take down Tirek. At the end, Discord was accepted by other ponies.)