Burt the Bashful

Burt the Bashful is the first mini-boss you meet in Super Mario World 2: Yoshi's Island, in World 1-4. To defeat him, the yellow Yoshi must hit him with 4 or 5 eggs, each time ducking underneath the platform in crevices to keep Baby Mario protected. When he is defeated his pants fall off and he turns red, eventually imploding. As stated on the official Yoshi's Island: Super Mario Advance 3 website, "you can't fight while your pants are falling down!"

Trivia

 * The first boss (actually a pair of bosses) in Yoshi's Island DS, the Burt Bros., may be a reference to Burt the Bashful.