From the Super Mario Wiki
Harry Hedgehogs are small, quick-moving blue hedgehogs that appear in Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3. If Yoshi gets too close to one, the hedgehog swells to a huge size, its spines sticking out. They cannot be eaten while in their defensive position. They usually appear in cavern levels; the 6th level of World 3, The Cave Of Harry Hedgehog, is named after this enemy, and prominently features the hedgehogs.
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