Birdo

Birdo is a recurring character appearing in the Super Mario Bros. Series and spinoffs, first found in Super Mario Bros 2. It's seen many times, but no traces of Birdo were ever found in Super Mario 64. In some personalized copies of the game, however, a Birdo can be found, often acting as a boss.

Super Mario 64 manifestations
In some personalized copies, Birdo can be found on the roof of Peach's Castle, often replacing Yoshi. It might also appear in the third floor. In most cases, it would challenge the player with a Boss Battle. The ways to defeat it vary, but in most cases the player will need to Sweep Kick or Ground Pound on it triple times. Its attack methods are egg-spitting and fire breaths. Upon defeat, it would either drop a Power Star, a key, or a warp to a hidden level. Entering the level would reset the game, possibly due to the level not being present in the files.

Theorizing
It is theorized that this boss was once planned to appear on the roof or the third floor, but was scrapped due to its low purposes on gameplay. It might also be created by the Personalization A.I. for players that needs a challenge. In some cases, Birdo is a recolor of Yoshi, implying that it's created by Enemy Discoloring. Nintendo might had used birdo as an example to experiment on Behavior Changing, but removed it before releasing to hide the AI.