From the Super Mario Wiki
In Super Mario World 2: Yoshi's Island, the Official Guide from Nintendo Power describes how the Yoshi franchise has different enemy classes. Of those, one of them is Ucantia Defeatus, which is a pseudo-scientific name corruption of the phrase "You can't defeat us". 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. This classification also refers to the game's bosses.