Grrrols are small, spiky Thwomps that appear in New Super Mario Bros. U and New Super Luigi U. They appear in the levels Underground Grrrols and Grinding Stone Tower. They attack simply by rolling in a direction and changing directions when they hit a block or obstacle.
After being in New Super Mario Bros U and Luigi U they then went onto being in Mario Party 10 and Super Mario Party as enemies in minigames, such as Hop, Drop and roll in Super Mario Party 10 and Get over it and Follow the money in Super Mario Party.
The only way to defeat one is with a Starman powerup.