"The historians had slipped up. In addition to the five supporting Elements that accompanied Twilight to the Everfree Forest, there was also a Sixth Element... and it wasn't Magic!"
A combination of a backstory for Derpy and Dinky, and a look at the events that took place in Ponyville during Nightmare Moon's return. Also answers the question of why everyone in Ponyville so readily puts up with the wonderfully klutzy Derpy, no matter how much damage she does.