Munchers are a type of Piranha Plant found first in Super Mario Bros. 3. They are small and black, and can sometimes be incased in a block of ice, which is safe to walk over. They appear in large groups taking up space between the space of two pipes. If stepped on Mario will get hurt or die. Using Yoshi in Super Mario World you can walk across them and not get hurt. In the Pipe Maze in Super Mario Bros. 3, there is a level where there is a large line of Munchers and the only way to get across them is to use a Starman. Using a POW Block turns them into Coins and it's the only way to defeat them.
In New Super Mario Bros. Wii, Munchers are found only in World 9 in level 9-7, incased in a block of ice like in Super Mario Bros. 3, but note that after the ice is melted, the Muncher cannot be frozen again. Unlike in Super Mario World, Yoshi is harmed by the Muncher. This was the only game where Mario cannot walk over the Munchers.