Morpha

Giant Aquatic Amoeba: Morpha is the boss of the Water Temple in Ocarina of Time. Created by Ganondorf once he acquired the Triforce of Power, it has complete control over the water in the room in which it resides, moving swiftly through it and creating long amoeba-like "limbs". This monster is responsible for the receeded state of Lake Hylia as well as the frozen condition of Zora's Domain.

Morpha itself is not the transparent muscle-like tentacle as its official artwork suggests, but the vulnerable red nucleus often safeguarded under the muck of the thick liquid. This nucleus can manipulate the water in any way it likes, creating one (and eventually two) large water tentacles as the battle goes on.

Strategy
Morpha is a very motile boss, meaning Link must keep moving during this battle, especially towards the end, if he is to survive. The nucleus will churn the water below and rise within the center of one of its created tentacles. While the amoebic liquid is impossible to penetrate when in large concentrations in the pool, whenever a single arm rises and latches upon the land, the liquid becomes thin enough to pierce with the spike of the Hookshot. Link must hook the nucleus, and slash at it while it is on land during its Critical Point, the time in which it is entirely vulnerable and clear of any ameobic fluid. Morpha will eventually adapt to the battle and grow a second tentacle, using it to distract Link from latching on to itself again. Just like with the single tentacle, the second one will grab Link, swing him about, then chuck him at the spikes at the edge of the arena, causing surplus damage. Though the battle becomes a bit more urgent as the second tentacle grows, the technique used to defeat Morpha remains the same: Link must continue to hook the nucleus and smash it, continuously, while it is bouncing about on land. After injuring Morpha's nucleus many a time, the core will burst in a spray of water. The tentacle will cling to life as long as it can by using up all the water in the central pool, and by attempting to drain and escape through the ceiling. Once it sucks up all the water in the pool, the tentacle will culminate in a single water droplet on the ceiling and drop to the floor, splattering in all directions.

Link's rewards for his victory is a Heart Container and the Water Medallion from Ruto.

Trivia

 * Din's Fire is capable of evaporating Morpha's tentacles.
 * Morpheel is a reference to Morpha, both in the way its first stage is defeated and in name.
 * Morpha can be very easily defeated by using the Longshot to pull it, and keep doing so when approaching the corners of the room to avoid it escaping. Link must repeatedly slash at it while trapped in the corner to defeat the boss quickly, and often injury-free.
 * After Morpha is defeated, Lake Hylia is restored. However, Zora's Domain remains frozen.