So after watching the season 4 finale again and the season 9 finale I realized Discord can take different forms if he wants, similar to changelings so I thought why not make use of this. Eris can take the form of anypony she wants, why she chose Fluttershy in this case I can't say. Without spoilers which I'm not willing to give anyone as of right now. You can figure out why if you watch my series when it comes out though.

Oh and.. if I got any tags wrong, I apologize I don't know if there is already a concept like this/ tag. So if anything seems wrong feel free to fix it thank you '