From the Super Mario Wiki
Horsetails are items found in an area of Petal Meadows in Paper Mario: The Thousand-Year Door. Just before reaching the area with the bridge, coming from the Rogueport pipe, there is a Candy Cane that, when hit with a hammer, sinks down and bounces back up. After hitting the cane 10 times, a horsetail comes out. This can be done several times, after exiting and re-entering the area. They heal 3 HP.
Horsetails are also in Super Paper Mario, where they restore 7 HP and cures poison, and are usually dropped by Putrid Piranhas. They could also be bought in the Itty Bits in The Dotwood Tree for 18 coins.
Names in Other Languages
||Cola de Caballo