Grrrol

Grrrols are spiked stone enemies debuting 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 hit the playable characters, and bounce off the walls, themselves, and other solid objects. These enemies resemble circular Thwomps with rolling red eyes and a big grin with one tooth missing. The spikes they are surrounded by subtly alternate between lighter and darker shades of gray.

New Super Mario Bros. U
Grrrols first appear in New Super Mario Bros. U where like Thwomps, Grrrols are invincible enemies, and only the Super Star has an effect on them, as does hitting the Item or Brick Block they are on from below, making them slightly weaker than Spiked Balls. Grrrols are spawned by specific pipe-like walls, which can make infinite units. They can also defeat any enemy they roll into.

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.

New Super Luigi U
The Grrrols return in New Super Luigi U, appearing in the Layer-Cake Desert stage Underground Grrrols as the main obstacles. They also appear in Stone-Snake Tower of Soda Jungle.

Mario Party 10
Grrrols reappear in Mario Party 10 as obstacles in the minigame Hop, Drop, and Roll. They are periodically produced by a device on the ceiling, which moves across the screen. After falling on the ground, Grrrols move in a direction until they fall off the ledge. The characters are required to jump over the Grrrols and not get hit, otherwise they lose the minigame.

Super Mario Party
Grrrols reappear in Super Mario Party, where they appear as obstacles in the minigames Get Over It and Follow the Money, where in the former they must be dodged by jumping over them, and in the later they must be dodged by moving around them. A Grrrol, along with a Wiggler and a group of Chargin' Chucks, appears in Rattle and Hmmm, where it and the other two cause the player's Joy-Cons to rumble. Later, the player must match the subsequent rumbling to the Grrrol or the other two. They may appear as one of the images in Absent Minded.

Trivia

 * Their name is a pun on grrr, a growling sound, and roll, their attack pattern.