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. It's originally a normal Burt until Kamek uses his magical powers on him to make him grow big. 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, ashamed, 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.
 * Burt the Bashful makes a cameo appearance in the Nintendo Monopoly.