Chop Chop (called Piranha in the Japanese Manual), found in Aquatic Ruin Zone, is a blue mechanical fish found only underwater that swim faster when you approach. They don't have any fighting methods so it's best to simply jump on their heads to take them out.
36DAC Chop Chop sprite offset
89B9A Chop Chop sprite art (Nemesis Format, 24 blocks)
91 Sprite function