Klepto
Klepto is a large buzzard who attempts to steal Mario's hat in Shifting Sand Land during the events of Super Mario 64. 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.
When Mario first meets Klepto, he is holding a Power Star. Needing the star, Mario knocks the item out of the bird's grip and claims it for himself. Perhaps Klepto's obsession to steal Mario's hat stems from the fact that Mario stole his Power Star.
Klepto has the same Power Star and purpose in Super Mario 64 DS. He also appears in another level, the Sunshine Isles, in the toy room. He makes a final appearance in Tiny-Huge Island(when you're big.) He is carrying, as usual, a star - except this time it's a Silver Star.
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 Party 4. He is used to transport characters to different spaces on the game board. In Mario Party Advance, a Klepto guards a treasure chest with a star and you have to guess his favorite number. He gives it away by saying "What was that for for for for?" when you guess wrong. So the code is obviously 4444.He then gives you the star.
Buzzar from Paper Mario might be related to the Kleptos.
Klepto gets his name from the term kleptomania, the inability to resist the impulse to steal.
