Harry Hedgehog

Harry Hedgehogs are small, quick-moving blue hedgehogs that appear as enemies in the Yoshi's Island series.

Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Harry Hedgehogs make their debut appearance in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. They continuously and quickly move between two sides of a platform. If Yoshi gets too close to one, the hedgehog stops in its tracks briefly and swells up, its spines sticking out. They cannot be eaten while in their defensive position, but can be defeated using a fire watermelon, and are otherwise edible and vulnerable to any attacks in their normal state. They usually appear in cavern levels; the sixth level of World 3, The Cave Of Harry Hedgehog, is named after this enemy, and prominently features the hedgehogs. They appear in another level in Yoshi's Island: Super Mario Advance 3, in Go! Go! Morphing!

Yoshi's New Island
Harry Hedgehogs reappear in Yoshi's New Island, where their spines are now purple instead of blue. They prominently appear in Harry Hedgehog's Labyrinth and act the same as previously.

Super Mario World 2: Yoshi's Island

 * Shogakukan guide: 近づくと、ハリを出してからだを大きくするハリネズミ. 小さいうちにかたづけたい. ('A hedgehog whose needles will emerge and make its body appear larger when approached. It is best to deal with it while it is small.'')
 * Player's Guide: Strictly a cave-dweller, this small creature puffs up his spiny back when strange dinosaurs approach.

Yoshi's Island: Super Mario Advance 3

 * Shogakukan guide: 近づくと、ハリを出して体を大きく見せる. 大きくなっているときは、タマゴで倒すことも飲みこむこともできないので、小さいうちにかたづけたい. (When approached, their needles will emerge and make their body appear larger. When they are large, they cannot be knocked down with an egg or swallowed, so it is best to deal with them while they are small.)

Trivia

 * Harry Hedgehogs are possibly a reference to Sonic the Hedgehog, since they are blue and move very fast. During the 16-bit era (which happened to be the time that Super Mario World 2: Yoshi's Island was released), Sonic's company of origin, Sega, often mocked the works of Nintendo in various ways (as Nintendo and Sega were business rivals). The Harry Hedgehog may be a retaliation to these mockeries, which may also explain its change in color for its latest appearance. However, in official artwork, Harry Hedgehog has always been purple.