Monkey
Monkeys are recurring bit part players in Zelda games. They are often mischievious and skittish, but come through in a pinch. They typically resemble chimpanzees, macaques or other small "new world" monkeys. Most are not enemies, but either help or are helped by Link.
A Link to the Past
A chimpanzee named Kiki (Japanese for "merry") suddenly appears behind Link when he exits the canopy maze before the Palace of Darkness in the Dark World. He asks you for 10 rupees, after which he will follow you. When both Link and Kiki reach the palace entrance, Kiki will open the door for you for 100 rupees. If Link pays him, Kiki looks around for a bit, then jumps on the roof and hits a switch, opening the door. If Link declines to pay either time or is hit by an enemy, Kiki will run off, and Link must return to the maze to find him.
Link's Awakening
At Toronbo Shores, a monkey throws coconuts and bombs at Link. He is an enemy, and can be jostled out of the tree with the Pegasus Boots.
Northeast of Kanalet Castle is a friendly (but confused) monkey also named Kiki[1], standing in front of the moat. If Link gives him the banans he gets from Sale's Banana Shop in the trading sequence, he then calls an army of monkeys that constructs a bridge for you. Also, they leave behind a stick, the next item in the trading sequence.
Majora's Mask
This monkey can be found in the Woods of Mystery. He guides you through the passages and finally to the injured witch whom you have to heal with a special red potion.
Monkeys also appear in the part of the storyline before Woodfall temple. A group of his monkey friends accost your for your help before you enter Deku territory, telling you that their friend has been accused of kidnapping the Deku Princess, and is about to be roasted as punishment. As a Deku, you must sneak around to the other side of the throne room and talk to the tied-up monkey, who teaches you the Sonata of Awakening. You can use this to defeat Woodfall Temple.
After recovering the Deku Princess, Link brings her back to the Deku King, who frees the monkey (after a stern talking-to by his daughter).
Twilight Princess
Monkeys play a very important role in the first dungeon of Twilight Princess. Four monkeys are captured at various points within, and you must set them free to progress. In return, the monkeys assist you by climbing on ropes and swinging you to far away platforms.
Also in this temple is a large baboon called Ook. He has been turned evil by a parisitic arachnid (possibly from the Twilight) and throws the Gale Boomerang at you from columns situated around the room. If Link rolls into the columns, he can knock Ook off, making him vulnerable. Once defeated, Ook loses his parasite, and drops the Gale Boomerang for Link to pick up. Later, Ook helps you defeat the Diababa by swinging back and forth with a bomb in its grip, allowing you to use the boomerang to blow it into the boss.
Monkeys are seen in a few other places in Faron Woods, either in need of help or helping you. They appear to be intelligent and friendly towards humans. One monkey has a flower in its hair, which may mean its either a female, or an authority within the monkeys.