Gohma

Gohma, the Parasitic Armored Arachnid, is the boss of the Great Deku Tree's dungeon in The Legend of Zelda: Ocarina of Time. She is the first boss Link encounters, and thus fairly easy to defeat. Her eye is her weakness.

In-game
Gohma is a parasite inside of the Great Deku Tree who is draining him of strength and slowly killing him. The Great Deku Tree calls Link to come defeat her, and after making his way through the dungeon, he encounters Gohma and must defeat her. Her weakness is her eyeball, and Link must dodge her attacks and hit the eye with a Deku Seed using his Fairy Slingshot to stun her. There are a few other methods of stunning her by hitting her eye, such as a Deku Nut, but once stunned, Gohma lays on the floor for somewhere around three to eight seconds, at which point Link can hit her eye with his sword or a Deku Stick. After being hit like this at least once, Gohma climbs on the ceiling and lays eggs to aid her in battle. When on the ceiling and her eye turns red, Link can hit her down with the slingshot, but she is not vulnerable when her eye is green or yellow. Once stunned and attacked many times, Gohma dies, dropping a Heart Container. Unfortunately, the Great Deku Tree still dies, but not before he entrusts Link with the Kokiri Emerald.

Trivia

 * Gohma is presumably female because she lays eggs and has children throughout the fight.