I'm positive this can't be original, but this was the first time I have done it, so it's original to me. :)
All you need is a ponytail, a topsy-tail, and a couple bobby pins.
1. Start with a simple ponytail, then use the topsy tail and pull it through the top. (So the loop is at the bottom, and the point is going up through the hair.) This picture is pulled through once.
2. Repeat! Just pull it through again, leaving this cute little bump. I just pinned the ends in a few places, and it was good to go. It looked great throughout the day, and took maybe 3 minutes total. I think it would have looked much cuter with a bow or something covering the "part" of the hair right above the ponytail, but Em's against bows at the moment. :)