The Prissy Pony's Padding is a My Little Pony fanfiction by Twinkie Hoof.
WARNING: Diaper and infantlism content, please don't complain or read if you don't like that. You know, so just don't read.
Synopsis from FIMfiction: "Octavia Philharmonica is refined, classy and elegant. She also has a secret involving things most ponies outgrow quickly. Unfortunately for her being in the back of her closet in a thick oak chest with a large padlock on it is not enough to deter a curious Vinyl Scratch.
Vinyl, however, is more taken with her friend's hobby than she expected and decides Octy needs to get over her shyness and embrace it. Even if she needs a little encouragement."