From the Super Mario Wiki
Mildes are small, pink ball-like creatures that appear in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. They march around in lines and are most common in Marching Milde's fort. Like balloons, they pop if a Yoshi steps on one, the pop knocking back any enemies that are close to them. A few of them are very large and have to be Ground Pounded to be defeated.
Mildes reappear in Yoshi's New Island.
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