Morpha

Giant Aquatic Ameoba: Morpha (水棲核細胞モーファ)is the boss of the Water Temple in The Legend of Zelda: Ocarina of Time. The boss' nucleus is able to manipulate the water in a pool, making it able to grab, constrict, and toss enemies or lash out at Link. It is a large, ameobic creature consisting of a main nucleus surrounded by purple cytoplasm.

Story
Not much is known about the Morpha, but apparently it was the cause of Lake Hylia drying up as well as freezing Zora's Domain. After Link defeated Morpha, the water rose again in Lake Hylia and Link recieved the Water Medallion, awakening Princess Ruto as the Sage of Water.

Battle
The only way to defeat the creature is to destroy its nucleus. Since it never leaves its pool, Link must reel in its nucleus with his newfound Longshot. During the battle, Morpha attacks by creating large tentacles of the water in the pool used to grab Link. If Link falls into the pool, Morpha (who isn't affected by the water's resistance) will continue to slam into him until he manages to get back up on land.

Morpha's tentacles cannot reach Link if he is standing in one of the room's corners. Another fast way of defeating Morpha is to try to trap its nucleus in the corner, allowing Link to continue to hit it without the nucleus escaping back into the water.

Video
This clip was provided by lleeoo2010 M5wE7VXvmlw

Trivia

 * A similar boss named Morpheel appeared in The Legend of Zelda: Twilight Princess. It appears to be a reference to Morpha, as they have similar names, and the first phase of the battle against Morpheel involves pulling Morpheel's eye out of a watery tentacle and slashing it.
 * The water tentacles created by Morpha bears some resemblance to those created by the aquatic aliens in the film The Abyss.