Isn't that a Skullfish? Look, don't ask me about everything, all right?
That's a Skullfish. It's a hassle to fight, so either kill it with the first hit or just run away!
Skullfish live in the seas around Great Bay Region, as well as in the water just outside of the Goron Village after defeating Goht in Majora's Mask. They spawn at certain intervals within the Bay and swim towards Link, biting him if he does not raise his defenses. Zora Link's electrical shield attack can be used to defeat them. In Majora's Mask 3D, they can also be caught in the Ocean Fishing Hole by wearing the Captain's Hat.
Skullfish are found exclusively in the Water Temple in Tri Force Heroes. They viciously attack any Link they find swimming near them, and will leap out of the water to get to Links standing on raised platforms. To defeat them, they can be slashed with the Sword while they leap over a Link, or dredged from the water with the Water Rod. They can then be attacked as a normal enemy. In this appearance, they are green rather than bone-colored.
There is also a desert variant of this enemy called the Sandfish, which instead inhabits the sand of The Dunes, but behaves identically.
The following section is not part of the Zelda canon and should not be taken as such.
Skullfish can be found swimming in deep pools of water in Cadence of Hyrule. They move to the beat of the music and move every two beats, always crouching within the water for one beat before moving to an adjacent tile on the next beat. They will typically try to attack Cadence, Link, Yves, or Zelda at close range. They can be lured into shallow water, where they will beach themselves and flop around helplessly for seven beats before they jump back into the water. Occasionally, they will leap into the air before diving back into the water again.