A yellow mushroom is a fungus from a specific kind inside the mushroom species. It only exists in Rayman Revolution in a hidden area of the Fairy Glade, after performing a Rain Dance in a specific zone. This yellow mushroom shows up the words "Eat Me!" above, and when Rayman eats it, he shrinks, just like the effect of the Flying Blue Elf in the original Rayman game. By eating one yellow mushroom again, Rayman returns to his original size. This kind of ability exists as a power in Rayman Origins, after saving the Gourmand Land nymph, Edith Up.
The effect of this mushroom and the text "Eat Me!" is a reference to Alice in Wonderland.