From the Super Mario Wiki
Grindels Also known as Keith the Block are a type of Thwomps that debuted in Super Mario 64. They appear as mummified Thwomps, with a pair of small round eyes and a grin on their faces. They only appear inside of the Pyramid of the level Shifting Sand Land. They attack in a similar fashion to a Thwomp, floating up in the air vertically and fall over the ground, squashing anybody below them. Grindels can also move horizontally by "jumping" back and forth.
Grindels also appear in Mario Party 2, in the minigame Lava Tile Isle. Here, seven Grindels form a platform over a pool of lava. They move around to leave gaps, so that unaware players fall off to the lava.
Grindels appear once again in the Nintendo DS remake Super Mario 64 DS.
A similar creature is the Spindel, which is basically a cylindrical obstacle with the same appearance as the Grindel.
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