If the cover isn't enough of a dead giveaway, there are ponies in diapers in this fic. There will also be some elements of wetting and age-play involved. I hope you aren't offended by this.
Applejack is invited by Rarity to a special youth club for ponies to forget about their troubles and their stress.
However, once Applejack is about to join the group inside, she realizes there's an emphasis on the word 'Youth' in this youth club.
This fic was inspired by Twilight And Rarity Go To The Spa, by TheConflictedWriter. Be sure to check him out as well when you have the time.