Kokiri

The Kokiri (コキリ Kokiri) are forest-dwelling beings who live in the shadow of their patron deity, the Great Deku Tree. All Kokiri are green-tunic-wearing children, and have magical anti-aging characteristics. To date, they have only existed in the realms of Ocarina of Time and The Wind Waker, but are mentioned in passing in the game text of several games of the Zelda franchise.

Origins in Ocarina of Time
The Kokiri live under the leadership of Mido and the protection of the Great Deku Tree in the Kokiri Forest, deep within the Lost Woods region of Hyrule in Ocarina of Time. The Kokiri dress in green Kokiri Tunics, and have guardian and companion fairies who supervise them; these fairies seem to be gifts from the Great Deku Tree to the Kokiri. Every kokiri has a fairy.

Link was originally raised as a Kokiri, despite his Hylian heritage, based on a poignant move by his injured mother: she placed him in the care of the Great Deku Tree during the Hyrulean Civil War before her death in order to safeguard his life from the calamity beyond.

Lifestyle
The Kokiri were created by the Great Deku Tree and therefore, their homes are built not too far from him. It was said that the Kokiri die if they leave the forest. However, at the end of Ocarina of Time, the Kokiri are seen in Lon Lon Ranch along with other characters celebrating Ganondorf's defeat. Because of this, it is theorized that the story is of metaphorical nature, or at least one of general safety. If a Kokiri leaves the forest, they might be killed without the protection of the Great Deku Tree, and therefore eventually die if they do not return.

The Kokiri appear predominantly in Ocarina of Time, but also make one appearance in The Wind Waker: the Wind Temple guardian and Wind Sage, Fado, is a Kokiri; he looks and acts akin to his Ocarina of Time counterparts.

Theory
It is widely speculated that the Kokiri of Ocarina of Time return in The Wind Waker as the Koroks. Both are mentioned to be children of the Deku Tree. The Deku Tree also mentions that the Koroks "once took on human forms". Strangely, the Koroks can leave the haven that the Deku Tree resides in, whereas the Kokiri can not leave the Kokiri Forest. Note that is only speculated that the Koroks are the evolution of the Kokiri.

In Twilight Princess, because the Faron Province is populated by a large monkey tribe, it has also been speculated that the Kokiri evolved into such monkeys in the time between Ocarina of Time and Twilight Princess, especially since this very province houses not only the Forest Temple, but the Lost Woods as well, as did the Kokiri Forest region in Ocarina of Time. Besides local birds, these monkeys are the largest, most intelligent inhabitants of these areas.

Because of the lack of evidential fact and Nintendo confirmation of either connection, it is not possible to prove or disprove either of these theories.

Trivia

 * The lumberjacks in A Link to the Past are sometimes thought to be Kokiri.
 * The symbol of the Kokiri can be seen in Twilight Princess on the doors of the Forest Temple and on the arm of a monkey.