From the Super Mario Wiki
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Millstones are large indestructible stone wheels often featured in ruined areas. They mostly serve as obstacles that harm the playable characters by rolling onto them.
Donkey Kong Country
Millstones first appear in Donkey Kong Country where they only appear in two temple levels, Millstone Mayhem and Temple Tempest. Millstones are controlled by brown Gnawties, who sit in the center hole, and they either move on predetermined paths attempting to hinder the Kongs, or are stationary at first but start chasing them at a distance before stopping.
Mario Superstar Baseball
Super Mario Galaxy 2
Millstones appear in the Clockwork Ruins Galaxy from Super Mario Galaxy 2. They now take the shape of large stone gears and display rinds on the sides. Millstones fall in indefinite numbers onto inclined pathways and roll down towards the player. They feature cuts on their surface where the player can take shelter to avoid getting squashed, as this directly causes them to lose a life. There are also Millstones that do not roll down slants, instead rotating in one fixed place, allowing the player to use their rotation to scale the walls and platforms inside the Millstone.
Mario Kart 8 / Mario Kart 8 Deluxe
In Mario Kart 8 and Mario Kart 8 Deluxe, millstones appear as obstacles in the track Thwomp Ruins. They roll around certain parts of the track after 45 seconds in a race. At one point, a millstone rolls over a wall blocking a Glide Ramp, thus making it available for use. Unlike most hazards in Mario Kart 8, slowly driving into a roller does not make the racer tumble over.