Klepto

Klepto is a large buzzard who attempts to steal Mario's hat in Shifting Sand Land during the events of Super Mario 64. Klepto gets his name from the term kleptomania, the inability to resist the impulse to steal.

Buzzar from Paper Mario might be related to the Kleptos.

Super Mario 64 / Super Mario 64 DS
When Mario first meets Klepto in Super Mario 64, he is holding a Power Star. Needing the star, Mario knocks the item out of the bird's grip and claims it for himself; Klepto's subsequent obsession with stealing Mario's hat may stem from the fact that Mario stole his Power Star. Mario can attack the bird, knocking him out for short periods of time. Klepto, eventually, will regain his senses and continue trying to steal the hat. Without his hat, Mario takes more damage when attacked.

Klepto has the same Power Star and purpose in Super Mario 64 DS. He also appears in another level, the Sunshine Isles, accessible from the Rec Room. He makes a final appearance in Tiny-Huge Island, when the character is big. He is carrying, as usual, a star - except this time it's a Silver Star. Additionally, though Klepto only steals heroes' caps, he will attack and cause harm if he glances Yoshi in the vicinity even if Yoshi uses any of the plumber's Caps.

Mario Golf series
In Mario Golf: Toadstool Tour many Kleptos can be seen flying in a desert stage. This confirms that Kleptos are a species, and that maybe the Kleptos Mario sees in each game are different from the one in Super Mario 64.

Klepto also appears in Mario Golf: Advance Tour in the back of the Marion training grounds, in which the player must rescue a Koopa hanging from its claws.

Mario Pinball Land
In Mario Pinball Land  a group of four Kleptos can be seen flying around the pyramid in the Shifting Sands world. If Mario defeats them, he gets a star.

Mario Party series
Klepto also appears in Mario Party 4. He is used to transport characters to different spaces on the game board.

In Mario Party 5 it has its own orb, where, if landed on, Klepto will appear and take the character to another character's space determined by roulette. In the instruction guide, Klepto is called Clepto.

In Mario Party 6, Klepto can be seen flying in the background of Pokey Punch-Out.

In Mario Party Advance, a Klepto guards a treasure chest with a star and the player has to guess his favorite number. He gives it away by saying "What was that for for for for?" when the player guesses wrong. So the code is obviously 4444. The player gets the Star from the Klepto after inputting the correct code.

Klepto makes a small cameo in Mario Party 8 in the minigame contest Test for the Best, where he holds the 5000 points sign when the player reaches it.

Trivia

 * Klepto bares a resemblance to Necky from Donkey Kong Country.