From the Super Mario Wiki
Loch Nestor is a puffer fish enemy encountered in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. It only appears in stages where Yoshi must morph into a submarine. If it is hit by a torpedo, it simply grows two times larger and chases Yoshi. After it is hit three times, it explodes.
Loch Nestors reappear in Yoshi's New Island.
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