From the Super Mario Wiki
A Smolderin' Stu is a type of Stu that appears in the game Super Mario Sunshine. It is distinct from Strollin' Stu due to its ability to be covered in flames, yet be apparently unharmed. Mario will catch fire if he touches the flaming enemy. If the flames are removed, it can be seen to have a reddish skin color. They are very rare, only appearing at Pinna Park in the "Mecha-Bowser Appears!" mission and Sirena Beach in the "King Boo Down Below" mission.
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