Yoshi

Yoshi is a recurring character in the Super Mario series and the protagonist of his own Yoshi series. He's a frequent ally to Mario, assisting him since he was a baby, as seen in the Yoshi's Island games. His appearance in Super Mario World was met with mass approval, leading to him frequently appearing in other Mario games and spin-offs.

Yoshi cameos in Super Mario 64 on top of Princess Peach's Castle. Upon getting 120 stars, Mario can use the cannon to shoot himself onto the castle's roof. Upon talking to Yoshi, he will grant you 100 lives and a specialized triple jump as a reward for fully completing the game.

He also appears as the first playable character in Super Mario 64 DS, awakened by Lakitu when the Mario Bros. and Wario go missing. He, and other colored Yoshis, are the only playable characters in the game's multiplayer mode.

Unused Yoshi Egg
In Super Mario 64, there is a Yoshi egg graphic that is unused in the game. From this, it is believed that Yoshi was going to have a more significant role in the game. The exact use of these eggs is unknown, but they could have been used for a side-quest of some sort.

Yoshi's Suicide
After talking to Yoshi on top of the castle, he will jump off the castle and disappear. This strange event leads many to believe that after talking to Mario, Yoshi commits suicide for some unknown reason. However, the iQue leaks refer to this subroutine as "yoshi_GoHome", implying that it's instead Yoshi leaving to return to his home.

Yoshi's Saddle
Yoshi, who appears atop the castle after collecting all 120 stars, has a different saddle design in Super Mario 64. Notably, the white rim that should be around his saddle is missing. The modelling of Yoshi's saddle implies that the white rim was supposed to be there, but was erroneously coloured red.

Trivia

 * The dialogue Yoshi has in Super Mario 64 has a typo, where he says "It that really you?" instead of "Is that really you?".