From the Super Mario Wiki, the Mario encyclopedia
Ucantia Defeatus is an enemy class in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. It is a pseudo-scientific name corruption of the phrase "You can't defeat us". Enemies in this class can be defeated only a certain way and cannot be eaten. They include Piranha Plants, Tap-Taps, Boos, Zeus Guys, Bandits, and the game's bosses.