A Link to the Past
Water Gibos are enemies that live in the Swamp Palace. They appear in the shallow water and once they have completely formed, they'll bounce around the room until they hit their target or simply disappear. Link can temporarily defeat a Water Gibo with a wide variety of weapons. A simple slash with his Master Sword or a hit from his Boomerang will do the trick. However, this will just cause the Water Gibo to form once again from its home base moments later.
There is only one way to permanently defeat a Water Gibo so that it doesn't respawn. While the Water Gibo is bouncing around the room, Link can hit it with the Ice Rod or use the Ether Medallion to freeze the enemy. Link then must use the Magic Hammer to pound the enemy, defeating it for good. Even after the Water Gibo has been frozen, if it is defeated with any weapon other than the Magic Hammer, it will still reappear. If Link doesn't freeze the Water Gibo and instead just hits it with a direct hit from the Magic Hammer, the Water Gibo will still respawn. Thus, it is required to first be frozen, and then hit with the Magic Hammer.
Water Gibos also appear in the Palace of the Four Sword for the Game Boy Advance version of A Link to the Past. After Link has defeated all of the Green Bari surrounding the Orange Arrghus, it summons a swarm of Water Gibos while speeding across the shallow water.
A Link Between Worlds
Like in A Link to the Past, Water Gibos are enemies that are found in the Swamp Palace. They appear in shallow and deep water, and once they have completely formed, they'll bounce around the room until they hit Link or simply disappear. Link can use a multitude of items to defeat these enemies like the Hammer, Tornado Rod, or a Golden Bee. However, this will just cause the Water Gibo to form once again from its home base moments later. Unlike in A Link to the Past Water Gibos can only be temporally defeated and not destroyed permanently. A Water Gibo causes Link half a heart of damage if he comes into contact with one of them.