From the Super Mario Wiki
In Super Mario World 2: Yoshi's Island, the Official Guide from Nintendo Power describes how the Yoshi series has different enemy classes. Of those, one of them is Ucantia Defeatus. Enemies in the class Ucantia Defeatus can be defeated only a certain way. Common examples of these enemies include Piranha Plants, Tap-Taps, Boos, Zeus Guys, and Bandits. Including the Bosses.