From the Super Mario Wiki
“Ook...oookiii! (I used your capsules!)”
Ukikis, also known as Ukkikis or simply monkeys, are a species of primates found in the Mario series. A similar species of monkeys called Grinders originated in the Yoshi series of games, and possibly inspired the creation of Ukikis. Interestingly enough, possibly as a mistake, Yoshi's New Island refers to Grinders as Ukikis.
Super Mario series
Super Mario 64
In Super Mario 64, two Ukikis inhabit Tall, Tall Mountain. One can be seen following Mario around and doing tricks. However, if Mario grabs him, he will steal Mario's hat and wear it. To get it back, Mario has to grab him again; this is made difficult by the fact that Ukiki now runs away from Mario. In the second mission of Tall, Tall Mountain, Mario has to catch a different Ukiki on the very top of the mountain; when caught, the monkey will jump on a cage with a star for Mario, making it accessible.
Super Mario 64 DS
In the remake Super Mario 64 DS, if any character was wearing another character's hat, the monkey would take it and the character would transform back into himself. A glitch allowed Wario to pick up Ukiki using the Metal Cap and could even carry him into the water.
Mario Party series
Mario Party 4
Mario Party 5
In Mario Party 5, they were revealed to also come in other colors: gray, blue, red and very rare gold ones. They had their own capsule as well: when activated, the Ukiki would use all of another character's capsules.
Mario Party 6
In Mario Party 6, Ukiki appeared in Castaway Bay hosting a special coin collecting minigame. They also appeared in various minigames such as Snow Brawl, where they act as allies to the solitary player, and Strawberry Shortfuse, where they deliver plates with either cake or explosives. They also appear in the Memory Quiz, the Variety Quiz, the Comparison Quiz and the Counting Quiz of Speak Up
Mario Party 7
Mario Party 8
In Mario Party 8, Ukikis appeared in DK's Treetop Temple, where they would roll down the stage on barrels, taking ten coins from anyone in their path. Mechanical Ukikis appeared in the minigame Flip the Chimp. They appear in the battle minigame Snipe for the Picking where there are two Ukikis turning cranks. They also appear in the duel minigame Cardiators on one of the cards. When the card is chosen, they come charging in to attack the opposing player.
Mario Party Advance
In Mario Party Advance, a female Ukiki named Akiki (who happens to be the niece of an individual Ukiki who is named "Ukiki") appears. The actual Ukiki is the mayor of the Jungle Area, and lives at a house in the area. However, after arriving at his house, he was supposedly kidnapped, the kidnapper leaving a note stating his location in code, saying bIg teAM cAT leaTHEr fLAKE blarmpot (saying "I AM AT THE LAKE" if lowercase letters are removed). However, after arriving at Loch Dorrie, Ukiki states that he wanted to surprise Akiki, but he never thought it would become so serious. He then left to explain to Akiki what had happened, but not before giving the player the gaddget Mini Maze.
The ending states that Ukiki resigned as mayor, and is still Akiki's friend.
Ukikis also appear in the minigame See Monkey?, which involves saving ten Ukikis from drowning.
Mario Power Tennis
Ukikis also have a place in Mario sports titles; like Mario Power Tennis, where they appear in the background of a court.
Dance Dance Revolution Mario Mix
Mario Baseball series
Ukikis also appear as part of the audience in the Mario Baseball series.
Mario Strikers Charged
They can also be seen in the audiences of Mario Strikers Charged.
Names in other languages