Pirogusu are enemies from The Legend of Zelda: A Link to the Past.
A Link to the Past
Pirogusu are fish-like creatures that only found in the Swamp Palace in the Dark World. They are only found in the parts of the palace with shallow water. Link can use his Master Sword to defeat these creatures in one swipe. However, they usually come out from sewers in the walls and an unlimited amount of them will keep coming out as they are defeated or if they swim off screen. Thus, they are better just avoided as opposed to trying to keep defeating them.
The water they travel in can kind of camouflage them, so Link always have to keep an eye out for them while he travel through the Swamp Palace. If a Pirogusu does happen to come into contact with Link, two full hearts of damage will be done.