Discord is the very embodiment/spirit of chaos, being the main antagonist of the My Little Pony: Friendship is Magic episodes "The Return of Harmony" and (indirectly) "Princess Twilight Sparkle".
The Return of HarmonyEdit
Part one of the episode opens with the Mane 6 experiencing some odd phenomenon such as the clouds raining chocolate milk and strange changes in the animal's physique. Princess Celestia calls them to her castle and explains that a great evil is upon them, one returning from the past. Discord, the god of chaos, returned decades after temporaily taking Equestria and being turned into stone by Celestia's and Luna's Elements of Harmony. Now that they are too old to wield the elements properly and had the power bestowed upon the Mane 6, Celestia attempts to give the elements to them. However, they are missing and the entity behind it is a returning Discord who taunts Celestia and rest with his manipulative psyche. Giving them a riddle on where the elements are hidden, Discord watches as the Mane 6 deduct the location at a hedge maze. Discord forces them to traverse through the maze's grassy interior without their powers and secretly turns them into corrupted jerks one by one. Applejack constantly lies, Pinkie is a fun hating bore, Fluttershy is a malicious brute, Rarity is a selfish fanatic, and Rainbow Dash is a disloyal traitor who retreats from the maze. Twilight is the only non corrupted one, and Discord reveals that the elements weren't in the maze in the first place. Ponyville is in utter chaos, but the Mane 6 find the elements in Twilight's house and attempts to put an end to Discord's tyranny. However, since Rainbow Dash is missing, the elements don't work and Twilight loses all hope before finally becoming corrupted like the rest of her former friends. She then opens her eyes, regains her strong hope, and manages to turn the Mane 6 back into the loving friends they originally were. Discord's chaos spree is put to an end and he is turned into stone yet again.