Originally introduced in New Super Mario Bros. the Mega Mushroom only made a cameo in Mario and Luigi: Bowser's Inside Story. It can give characters the ability to grow into great sizes.
In one of his attacks, Broque Monsiuer hits a ? Block to retrieve an item from it. One of the items that can come out is a Mega Mushroom. If this mushroom does come out, he grabs it, turning huge, and charges toward Bowser.