The Man and the Mare

Nightmare Moon didn't just sit around after Celestia banished her. Filled with a thirst for vengeance, she became determined to break out of her prison. She alters a powerful spell and casts it, hoping it will send her back to Equestria, but it backfires. She's still on the moon, but it's not HER moon. Mare in the Moon, meet the Man in the Moon.