From the Super Mario Wiki
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Roller Guys are a variant of Shy Guy that appear in Paper Mario: Color Splash. They ride around inside of a yellow and green striped ring, and they come in red, green, and pink. When they are inside their ring, Roller Guys cannot be attacked by jumps or hammers. They attack by rolling into Mario. After they are attacked a few times, the rings will break. They will act like normal Shy Guys once their ring is broken.
Roller Guys appear in Mossrock Theater, and some will ride on top of their ring and try to run over Mario. When on top of their ring, they cannot be battled.