Twilight Sparkle now knows how Celestia got the nickname 'Sunbutt', thanks to Luna — but when Celestia finds out, she decides to even the score.
After all, she isn't the only one with an embarrassing nickname and a story to tell, even if both were borne of tragedy...
Everypony thinks that Celestia is called 'Sunbutt' because of her cutie mark. Only Luna knows the real story, though, and now she's going to share it with Twilight.
(Sequel to 'The Origin Of Sunbutt'.) Now it's Celestia's turn to share a story with Princess Twilight...