Morpha

Morpha, the Giant Aquatic Amoeba, is the boss of the Water Temple in The Legend of Zelda: Ocarina of Time. As its title implies, it's an amoeba, but not the tentacle in the official artwork, but rather the small red nucleus inside. This nucleus simply surrounds itself with water to attack Link, but the water is not part of it as a life form.

In-game
Morpha is the evil that is responsible for draining Lake Hylia and freezing Zora's Domain. Link encounters Morpha at the end of the Water Temple, in a room where Ruto warns him about the strange nature of the water. Indeed, the water takes on a gel-like consistency, creating long appendages that are controlled by Morpha, the nucleus, trying to grab and attack Link. This is Morpha's main attack - making a tentacle and rising out of the water, trying to grab Link and fling him across the room. When one tentacle, with Morpha inside, rises out of the water, Link can pierce it with the Longshot to bring it to him, then slash it with his sword. If Link is in the water, the nucleus will not form a tentacle but will instead charge at Link in the water, which he can only escape by getting out of the water. As the battle goes on, Link still needs to latch onto the nucleus and slash at it, but Morpha begins to create a second tentacle that can also throw Link across the room into the spikes (though if Link stands in the corner between the spikes it cannot grab him). The battle gets faster and more difficult with the two tentacles, but Link still must grab Morpha, the nucleus, from the water appendage, and slash it with his sword while it bounces on land. If Link can trap Morpha in a corner and slash away, Morpha won't be able to escape and can be defeated much quicker. Once the nucleus has been damaged many times, Morpha perishes, trying to survive by sucking up all the water in the room and attempting to go through the ceiling, before falling to the floor and splattering. Link is rewarded with a Heart Container and the Water Medallion by Ruto.

Trivia

 * Before Ocarina of Time's release, Morpha was referred to simply as "Water Spirit" on Nintendo's website.
 * Its name comes from the Greek word morphe, meaning "form" or "shape".