Rotten Mushroom

Rotten Mushrooms are purple, skull-shaped mushrooms that appear in Super Mario Maker 2. They replace 1-Up Mushrooms when night mode is activated, but only for the Ground course theme. Unlike most mushrooms, Rotten Mushrooms actively pursue the player, even jumping up ledges up to two blocks high to get to them. However, if the player is under the Super Star's effects, Rotten Mushrooms will evade them. Touching a Rotten Mushroom powers down the player, similar to a Poison Mushroom, making it a hazardous obstacle. Their skull-like design and role as an "evil" counterpart to "good" mushrooms makes them comparable to Poison Mushrooms, Zombie Shrooms, and Ghoul Shrooms. The Super Mario World version of the Rotten Mushroom resembles Bowser's rain of fireballs from said game.

While Rotten Mushrooms can only be placed in Overworld night areas, they can be brought into other themes if Yoshi grabs one with his tongue and travels through a warp pipe to a non-Ground night area. They display unique behaviors in other themes, despite the fact that they cannot be placed in them outright. As Yoshi is only featured in the Super Mario World and New Super Mario Bros. U game styles, they cannot be seen outside of Ground night courses in the Super Mario Bros. and Super Mario Bros. 3 game styles.
 * They behave as normal, only upside-down, in the Underground night theme
 * They are slowed by water, but jump three blocks high instead of two in the Forest and Underwater themes
 * They bear a small circle of light in the Ghost House and Underwater night themes
 * They are affected by low gravity in the Sky and Airship night themes
 * They are slowed down by lava and poison in the Castle and night Forest themes, respectively
 * They are unaffected by the wind in the night Desert theme

Trivia

 * Two Story Mode levels posted by World-Renowned Chef imply that Rotten Mushrooms are Super Mushrooms that have rotted over time and became vengeful.
 * The object filename for Rotten Mushroom is "Poison Kinoko," indicating that they are conceptually based on the Poison Mushroom.