From the Super Mario Wiki
Grrrols are a new species of Thwomp that first appear in New Super Mario Bros. U.
They are similar to Spiked Balls and Spiky Tromps from Super Paper Mario, as they roll around trying to crush Mario, bouncing the walls and themselves. Grrrols are put on-screen by specific pipe-like walls which can make infinite units. These enemies resemble circular Thwomps with rolling red eyes and a big grin with one tooth missing. Like Thwomps, they are invincible enemies, and only the Starman has an effect on them.
The first appearance of Grrrols is in a secret room in Stoneslide Tower from Layer-Cake Desert. They are found again in a Rock-Candy Mines tower, Grinding-Stone Tower, where they are main obstacles. This level also presents Grrrol's larger counterpart Mega Grrrol. These feature a rather toothy grin with cheeks which the smaller counterparts lack, though having the same traits.
These enemies reappear in New Super Luigi U. In Underground Grrrols from Layer-Cake Desert, as the name implies, they are important obstacles.
Names in other languages
|Onomatopoeia for rolling
||Pun on "grrr" and "roda" (wheel)
- Grrrols and Mega Grrrols have similar faces to Bristles.
- Their name is a pun on grrr, a sound made by similar species, Thwomps, and roll, their attack pattern.