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Kleptos are condors that first appear in Super Mario 64. Since then, he has continued to make infrequent appearances in the Mario series. His name is a pun on "klepto-", the Latin prefix for "theft".
Super Mario 64 / Super Mario 64 DS
In Super Mario 64, a Klepto is seen in Shifting Sand Land. He holds one of the castle's Power Stars, so Mario must reclaim it from his talons. Mario can attack the bird, knocking him out for a short period of time. The Klepto eventually regains his senses, and after losing the Power Star, he attempts to steal Mario's cap. If he leaves and still has no hat, it remains in the course he lost it in.
The Klepto has the same Power Star and purpose in Super Mario 64 DS. Another Klepto also appears in another level, the Sunshine Isles. A third one makes an appearance in Tiny-Huge Island, when the character is big. As with the one in Shifting Sand Land, he is carrying a Power Star. Additionally, though Kleptos only steals characters' caps, they can damage Yoshi.
Mario Pinball Land 
Mario Party 3
In the background of Spiny Desert, three Klepto-looking condors can be seen on the upper left side of the board.
Mario Party 4
A Kleptos also appears in Mario Party 4 on Shy Guy's Jungle Jam. Players can pay him five coins to be transported to the other side of the board, collecting coins in the process. In Paths of Peril, Klepto picks up players that falls off the stage.
Mario Party 5
In Mario Party 5, a Klepto has its own orb where, if landed on, it appears and takes the character to another character's space determined by a roulette. In the instruction guide, Klepto's name is erroneously spelled Clepto.
Mario Party 6
In Mario Party 6, Klepto can be seen flying in the background of Pokey Punch-out. The Klepto Orb returns as an orb that can only be placed on a space. Any player who lands on it will be taken back to the start of the board. In the Solo Mode board Thirsty Gulch, another Klepto can be seen flying around the board.
Mario Party Advance
In Mario Party Advance, Klepto guards a treasure chest with a star. When the player arrives at Klepto's Ruins, his quest involves attempting to open the chest by entering the 4-digit code, giving them the hint that it is also his favorite word. When speaking, he has a tendency to stutter and repeat the word "for," indicating what the code is. By guessing the correct code (4444) and opening the chest, Klepto rewards the player with the Gaddget Map Maker. The ending states that Klepto changed the code to the treasure chest.
Mario Party 7
A Klepto makes a few small appearances in Mario Party 7. Landing on one of the Green Spaces in Bowser's Enchanted Inferno! causes Klepto to grab the Star and move it to another location. Klepto can also be seen in cube form as the tops of totem poles on the Neon Heights board.
Mario Party 8
Names in other languages