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. 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 Super Star has an effect on them. Grrrols are spawned by specific pipe-like walls, which can make infinite units.
Grrrols first appear in a secret room in the Stoneslide Tower of Layer-Cake Desert. They are found again in the Rock-Candy Mines tower, Grinding-Stone Tower, where they are the main obstacles. This level also introduces the Grrrol's larger counterpart, Mega Grrrol.
The Grrrols return in New Super Luigi U, appearing in the Layer-Cake Desert stage Underground Grrrols as the main 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.