Horsetail

 are a cooking ingredient in Paper Mario: The Thousand-Year Door and Super Paper Mario, and are briefly referenced in the manual for Super Mario Bros.

Super Mario Bros / Super Mario Bros.: The Lost Levels
According to the original manual for Super Mario Bros., Bowser turned the citizens of the Mushroom Kingdom into horsetail plants and bricks. However, the English manual renders this as "field horsehair plants." They presumably are the ovoid tree-like plants seen throughout the game, which come in two sizes, and depending on the level's palette, can be either green or white, with both having a tan stem. These plants are also seen in Super Mario Bros.: The Lost Levels, along with several other games borrowing graphics from Super Mario Bros. In Super Mario Maker, Horsetails occasionally pop up in the ground theme in the Super Mario Bros. style.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Horsetails are items that heal 3 HP. They are found in an area of Petal Meadows. 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.

Super Paper Mario
In Super Paper Mario, Horsetails are items that restore 7 HP and cures poison. They are usually dropped by Putrid Piranhas. They can also be bought in the Itty Bits in The Dotwood Tree for 18 coins.