From the Super Mario Wiki
Retrieved from "http://www.mariowiki.com/index.php?title=Mega_Grrrol&oldid=2089027"
Mega Grrrols are found in the first Rock-Candy Mines tower, Grinding-Stone Tower, in New Super Mario Bros. U, and its equivalent level in New Super Luigi U, Smashing-Stone Tower. In both levels Grrrols are the primary obstacles. Mega Grrrols generally appear in confined areas and roll back and forth as obstacles, rather than coming from generators like normal Grrrols. Like normal Grrrols they can also smash through Brick Blocks.