Skullfish

Skullfish are recurring enemies in the Legend of Zelda series. Their appearance is that of reanimated skeletal fish. They are often found in groups and will attack as such.

The Legend of Zelda: Majora's Mask
Skullfish appear in the Great Bay region of Termina. They will start chasing after Link whenever he enters a body of water. They can be dispatched with the bio-electric energy field Link can activate in his Zora form. There is also a stronger kind of skeletal fish found in the game known as Desbrekos.

Some Skullfish also guard pools containing Zora Eggs inside the Pirates' Fortress. Since they cannot swim out of sight, these can also be defeated with one of Link's ranged weapons while he is standing at the side of the pool.

The Legend of Zelda: Twilight Princess
Skullfish can be caught with the Fishing Rod in the Lakebed Temple. When one is caught, it leaves a heart behind for Link to pick up. They are surprisingly easy to catch; in fact, if there are any Skullfish in a body of water, they are the only thing Link will catch, even if he has killed all existing Skullfish in the area. Furthermore, they are classed as "forbidden" and are not counted as part of Link's Fish Journal. When released, they are easy to kill as they just flop around on land. Interestingly, the game still keeps a hidden record of the biggest Skullfish Link has caught despite their forbidden status.