I was talking with another pony fan the other day when I mentioned I did not read much pony fanfiction. Of course later this week I opened google docs and noticed just how many fanfics I have read. I guess those small amounts really add up over time.
Rainbow Secret by *Don-KomandoRR on deviantART
A Terrible Secret by Sword of Squirrels was one of the first fanfics that I came across and really connected with. The premise, as suggested by the above image is that Rainbow Dash's main isn't quite what it appears to be.
I know this one is a bit of an older fic, but if you haven't read it yet I would very much recommend checking it out.