From the Super Mario Wiki
Munchers are an invincible species of Piranha Plants. Muncher is a obstacle. Unlike Nipper Plants, Muncher Plants come in black, depending on the location and are immobile. Even a Star or Goomba's Shoe can't defeat a Muncher, but it can prevent it from hurting Mario or Luigi. Only Mega Mushroom, Silver P-Switch or Statue Mario can defeat a Muncher.
Super Mario series
Super Mario Bros. 3
Munchers first appear in Super Mario Bros. 3 and are first encountered in World 5-3 of Sky Land where they appear in large numbers. Frozen Munchers can be identified by the black ice block in which they are encased. They are harmless when frozen and act like platforms, but if the player thaws a Muncher out with a fireball, the Muncher will become active again. If Mario hits a P-Switch, all regular Munchers will turn into coins. This is the only way to defeat them. If the player has Goomba's Shoe, they can hop along them without being harmed. In their artwork and the NES version, they are black; in Super Mario All-Stars, they are red, and appear as blue in ice stages.
Munchers that pop out of tiny pipes also appear in the Piranha Plant stages of the World 7. These Munchers are tan in colour with bright green leaves. In the NES version, Munchers are always black.
Super Mario World
Munchers appear again in Super Mario World, where they behave the just like they did previously, but they only appear in their black form. Having a Yoshi or been Invincible Mario can allow you walk on the Munchers without being harmed.
These Munchers are actually part of the environment rather than acting as a typical enemy, hence that they're not defeatable, similar to a Bill Blaster being part of the environment.
New Super Mario Bros. Wii
In New Super Mario Bros. Wii, Munchers appear as very rare enemies, since they only appear in World 9-7 where they are frozen in ice. They cover most of the floor and can be dangerous due to the surrounding enemies that hold fire in them such as Venus Fire Traps. The Venus Fire Trap's flames can burn the ice the Munchers are in, exposing them to the players. The players can also melt them if they have a Fire Flower. Once they are unfrozen, they cannot be frozen again, even if they are hit with a iceball from an Ice Flower.
Unlike Super Mario World, Yoshi can't walk on Munchers in this game; attempting to do so would harm Yoshi.
New Super Mario Bros. U
Nintendo Adventures Books
Munchers appear as scenery in Double Trouble, where it's mentioned Roy Koopa has parked the Doomship above a patch of them in the Mushroom Kingdom. In Leaping Lizards, some Munchers are a part of the first International Mushroom Games event, the obstacle course. If Roy's Magic Carpet is sabotaged by a Rocky Wrench, he will fall into a patch of the plants, though the Munchers leave him alone due to deeming his tough hide unappetizing.
In Pipe Down!, if Mario and Luigi follow Princess Toadstool out of her castle and to the Koopahari Desert, they have to zig-zag through patches of Munchers. The plants make a similar cameo in Dinosaur Dilemma, where they can be spotted prowling the Valley of the Koopas by Mario and Yoshi.
In Flown the Koopa, a patch of Munchers can be found in the desert surrounding a tree inhabited by Cheep-Cheeps, and will swarm Luigi if he tries to get his and his friends lost International Dino-Flying Derby tickets out of the tree; more of the plants can be found by the Mario Bros. in the swamp containing Magikoopa's Fortress.
Wario Land: Super Mario Land 3
Munchers also appear in Wario Land: Super Mario Land 3. Some of them retain their original color and are found early in the game. Other seem to guard much darker areas, like the ones found later in the game; these Munchers are not black. Although these characteristics of both types of Munchers apparently differentiate them, there is no real distinction between how they inflict damage on the player.
Characteristics and Traits
Munchers act like spikes, as touching them would damage Mario and Luigi. However, both Yoshis (in Super Mario World only) and the Goomba's Shoe can walk on them without harm. In a few levels, they spring from thin pipes. The only way for Mario or Luigi cross a patch of Munchers by himself is by acquiring a Starman or by carefully jumping to the gaps between Muncher patches.
Names in other languages