This story is a sequel to Rainbow Unicorns
Rainbow Burst -- a unicorn version of Rainbow Dash and Princess Celestia's personal student -- has been causing more mischief than usual around the castle (much to Blueblood's chagrin). Deciding the best thing would be to give Rainbow more responsibility and should spend some time away from the castle, Princess Celestia sends Rainbow to Ponyville to oversee the Summer Sun Celebration. While there, she and Spike meet some very odd (but familiar) ponies.
(I know, I have a problem. I couldn't resist. Anyway, enjoy.)