The phrase "mystery mushroom" refers to a type of mushroom whose identity or origin is unknown or unclear. It suggests that there is uncertainty about where the mushroom comes from or what its characteristics are. Full definition
12 new mystery mushroom costumes have been added as well. (nintendotoday.com)
Users can unlock new Mystery Mushroom costumes when they clear certain event courses. (gonintendo.com)
Super Mario Maker [Read]: Scan your amiibo to transform into an 8 - bit character costume based on the character with the new Mystery Mushroom item. (nintendo-insider.com)