From the Super Mario Wiki
A normal and big Cholesteron between games

A Cholesteron is a yellow enemy in Mario & Luigi: Bowser's Inside Story, and are found in Bowser's body in the Challenge Node. They can be encountered by choosing to participate in Cholesteroad. Instead of attacking, they serve as a punching bag to Mario and Luigi. The bros. can pound on them with Special Attacks to score points. Small Cholesterons are very weak and have little HP; however, there are much larger versions of Cholesterons that take more effort and time to beat. Cholesterons are never seen outside of the Challenge Node.