Burt the Bashful

Burt the Bashful (originally "Burt The Bashful") is the first boss seen in Super Mario World 2: Yoshi's Island; he is found in World 1-4. To defeat him, Yellow Yoshi must hit him with four or five 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., are based on Burt the Bashful.