Yoshi's Island

Yoshi's Island is an unused level found in some personalized copies of Super Mario 64. It is usually reported to be located on the Fourth Floor of Princess Peach's Castle, though others report the level to replace Lethal Lava Land as the 7th course in the game. It is described as a large island course with elements from the game with the same title. It's theme is described as the Flower Garden them from the original game, remixed to the N64 soundfont, while the cavern area uses the Underground theme from the SNES game.

Description/Geography
Yoshi's Island is fittingly based off the SNES game with the same title. Many enemies from the SNES game are found here, including Shy Guys. There are two main areas here, the first of which is the outdoors area, starting on an egg-shaped island leading to branching paths to the side areas of the course. On the egg shaped island, there is also a box. Breaking the box reveals a Yoshi egg, which then hatches. On the first time playing this level, Yoshi has the following text:

"Mario!!! Is that really you??? It has been so long since our last adventure! They told me that I might see you if I wanted here, but I'd just about given up hope! Is it true? You're on a mission to defeat Bowser and save Peach? It is? Well, I'm not just gonna stand here and be on standby for a Bowser counterattack, I'm joining you buddy, let's kick some butt!"

The player can then ride on Yoshi, and Yoshi's moveset is similar to that of Super Mario Sunshine and Super Mario Galaxy 2. With Yoshi, the player gains access to a variety of methods to earning the stars, being able to grab enemies with Yoshi's tongue and turn them into eggs Yoshi can throw.

The other main area is the cavern area, which can only be accessed through Yoshi, as the box blocking the entrance can only be destroyed with one of Yoshi's eggs.

Missions
The most commonly reported missions found in Yoshi's Island are a star found on top of an egg, where Yoshi is likely used for the first time, and a star in the cave, where the player must get 8 red coins. Yet another commonly reported star is finding five secrets hidden in seashells on the island shore, and the final commonly reported mission is a boss fight against Kamek, a Magikoopa introduced in the original Yoshi's Island.

The following text was also found for Kamek before battle:

"I've been waiting a long time for this moment, Mario. You and Yoshi have foiled me and Lord Bowser's plots so many times in the past and now I have you right where I want you. No running from me this time, it is a duel to the death. En garde!"

To beat Kamek, the player must dodge Kamek's magic attacks, and then counter by throwing eggs at Kamek when his defense is down. After defeating Kamek, the following text is reported:

"IMPOSSIBLE! How is it that you've beaten me? And with the help of that midget dinosaur as well? What kind of maniac are you? Ugh, looks like you win, and you get my star, but I'm warning you, if you think you stand a chance against Bowser, you are sorely mistaken."

Theorizing
There are several rumors of why Yoshi's Island was removed from the game. Some believed that it was because of a lack of time to fully develop Yoshi's moveset, whereas others speculate it was because of multiple glitches with Yoshi's eggs.

While the course itself would never make it into the game, Yoshi himself would make a cameo after the player collected all 120 stars (in non-personalized copies) and later appear in both Super Mario Sunshine and Super Mario Galaxy 2, having a big role in the latter game. There are also rumors that the Kamek fight could've been reworked into the Kamella fight in the original Super Mario Galaxy. On a side note, after unlocking Yoshi in this course, Yoshi is accesable in other courses and levels in the game including but not limited to Bob-omb Battlefield, Cool, Cool Mountain, Shifting Sand Land, Wet-Dry World, Tall, Tall Mountain, Tiny-Huge Island, and Rainbow Ride in non personalized copies, and Yoshi can appear in multiple courses in personalized copies, with some courses also having stars that require the use of Yoshi.