Horsetails are items from Paper Mario: The Thousand Year Door, which are found in an area of Petal Meadows. Just before you reach the area with the bridge, coming from the Rougeport pipe, there is a cane that, when hit with a hammer, sinks down and bounces back up. After hitting the cane 10 times, a horsetail comes out. You can receive the horsetail as many times as you'd like, after exiting and re-entering the area.
Horsetails revive 3 FP. Save one to make Healthy Salad with a Turtley Leaf, found in the nearby town, Petalburg (outside Mayor Kroop's house).
Horsetails also appear in the upcoming video game Super Paper Mario for the Wii, where they will restore 7 HP, and cure any poison that Mario or team may have.