Burt the Bashful is an enemy-type character that debuted in Super Mario World 2: Yoshi's Island. He is the first boss in the game. Formerly a regular Burt, Kamek used his magic to grow this boss to much larger proportions.
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!"
Burt the Bashful later reappeared as a boss in Yoshi's Woolly World.