Pirogusu are enemies from A Link to the Past.
Pirogusu are fish-like creatures that are 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 keep coming out as they are defeated or if they swim off screen. Thus, they are better just avoided.
The water they travel in can kind of camouflage them, so Link always has to keep an eye out for them while he travel through the Swamp Palace. If a Pirogusu comes into contact with Link, two full hearts of damage will be done.