From the Super Mario Wiki
A Spiny Fish is a small fish enemy found in the game Yoshi's Story, and they only appear in Level 5-1 of Yoshi Sea, Lots O'Jellyfish! They bear a small resemblance to Cheep-Cheeps, although they have purple scales along with huge, green eyes. They are armored with spikes; if a Yoshi eats them, he will lose some petals. They are exclusively found in underwater levels and behave much like Cheep-Cheeps, although they are quicker and will try to pursue Yoshi.