Wow! Looks like my one little find sparked quite a conversation.
But, from watching that scene with RD countless times, we know that her embarrassing hobby must be something quite feminine. It has been proven time and time again that RD is uncomfortable with feminine things like hooficures, trying on new dresses, etc.
It's gotta be something that tarnishes her image as a tough pony athlete. Being a ballet dancer is still a likely option because it's a hobby, something that goes against what RD is known for, and something that can be done in private. No pony is going to force you to perform if they never see you practice.
hinata2000100, I expect better proof if you hope to convince me and other people on this board about other possibilities. It's faulty logic if you claim that quote is true simply because it is. Don't beg the question, k?