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, Burt the Bashful's Fort. At the end of the fortress, Yellow Yoshi encounters a normal Burt, which Kamek grows to a massive size, turning it into Burt the Bashful. To defeat him, Yellow Yoshi must hit him with eggs six times; with each hit, Burt's pants continue to fall off. When he is defeated, his pants entirely 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.