From the Super Mario Wiki
Munchers ("Black Pakkun" in Japan) are an indestructible species of Piranha Plants. Unlike their white relatives, Nipper Plants, Muncher Plants come in black, depending on the location. Also, they are immobile.
 Super Mario series
 Super Mario Bros. 3
Munchers first appear in Super Mario Bros. 3. They are first encountered in stage 5-3 of The Sky 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. However, if Raccoon Mario or Tanooki Mario hits a Muncher with his tail or if Mario hits one with a Koopa, it will turn into an empty, used block. This was fixed in Super Mario Advance 4: Super Mario Bros 3. Though, 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. Yet in Super Mario All-Stars, they're red (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, all Munchers are black.
 Super Mario World
Munchers appear again in Super Mario World, where they behave the same as in their previous appearance, however they only appear in their black form. Like Goomba's Shoe, a Yoshi can walk on the Munchers without being harmed.
 New Super Mario Bros. Wii
In New Super Mario Bros. Wii, Munchers only appear in World 9-7 where they are frozen in ice. They cover most of the floor and can be quite 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. Unless the players are under the effects of the Super Star, they can't walk on the Munchers without being harmed.
Unlike in Super Mario World, Yoshi can't walk on Munchers in this game; attempting to do so would harm Yoshi.
 New Super Mario Bros. U
Munchers appear in New Super Mario Bros. U, they are found in Spinning-Star Sky. They are similar as from New Super Mario Bros. Wii, but they don't start out frozen this time. They behave the same way as they do in other games.
 Nintendo Adventures Books
Munchers appear as little more than 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 apart 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 find themselves having 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. They return to their original color, black, and they are indestructible, like in their previous appearances.
 Characteristics and Traits
Munchers act like spikes, as touching them would damage Mario and Luigi. However, both the Goomba's Shoe and Yoshis can walk on them without harm. In a few levels, they spring from thin pipes. The only way to cross a patch of Munchers is by acquiring a Starman or by carefully jumping to the gaps between Muncher patches.
There is often confusion on what a Muncher exactly is. They are about the size of a Nipper Plant, but are firmly planted in the ground like a Piranha Plant, making them immobile. In Super Mario Bros. 3 they have eyes, much like a Nipper Plant. However, in Super Mario World, they do not have eyes. They appear to be one of the many sub-species of Piranha Plants. In Super Mario All-Stars, these enemies are recolored, greatly resembling a Nipper Plant, and not black like the classic Muncher.