The character who is the spirit of chaos and disharmony and is later reformed is Discord.
Discord is a draconequus, a fantastical creature with mismatched animal features and reality-bending powers. He first appears in the Season 2 premiere, “The Return of Harmony,” where he escapes his stone prison and plunges Equestria into disorder. Princess Celestia and Princess Luna had previously defeated him with the Elements of Harmony.
Although Discord begins as a villain, Princess Celestia later releases him so Fluttershy can try to reform him in “Keep Calm and Flutter On.” Fluttershy’s kindness and insistence on friendship lead Discord to become a recurring, if unreliable, ally of the Mane Six. He remains mischievous and chaotic, but no longer acts as an enemy seeking to rule Equestria.
The other options are different antagonists: Nightmare Moon represents corrupted lunar power, Queen Chrysalis leads the changelings, and Lord Tirek steals magical energy. Discord’s name also echoes Discordia, the Roman personification of strife, and his design was inspired partly by Star Trek’s trickster character Q.