Yoshi

Yoshi is Mario's pet dinosaur that first appeared in Super Mario World. They appear in Super Mario 64 on top of the castle roof when the player obtains 120 stars, and will give 99 lives upon being talked to. However, Yoshi was originally intended to be rideable, and is unlockable in certain personalized copies, mainly via Sweet, Sweet Sanctuary. In Super Mario 64 DS, they are the character you begin the game with.

Unused Egg
In the files of Super Mario 64, there is an unused egg animation. It is believed this was intended for Yoshi when they were going to have a bigger role.

Yoshi's Saddle
In the model of Yoshi in Super Mario 64, there exists parts for Yoshi's saddle. However, these are colored red by accident.