Wario Land 64

Wario Land 64 is an urban legend relating to Super Mario 64, claiming to be an early prototype of the game as Nintendo feared that if they failed their big jump to 3D platforming with a mascot as big as Mario, it'd ruin his name. Thusly, they decided to use Wario instead.

Speculation
Wario Land 64 is speculated to be, as mentioned before, the earliest version of Nintendo's Super Mario 64, being even older than the July 29th 1995 build. Apparently, the infamous build was not the first version, modified to be a Super Mario game after being sold to Nintendo, instead. The legend claims that Wario Land 64 was created after a mysterious build created by an unknown Japanese organization that produced the technology responsible for Super Mario 64's ability to personalize itself to its player's likeness was sold to Nintendo. And then, afterwards, modified into being a Mario game.

Reportings

 * Everything in this section should be taken with a grain of salt. These are all unverified claims.

Developer Claim
"I'm sure most people are now aware of the fact that every single copy of Super Mario 64, from '96 onwards is personalised, strictly to appeal to the pleasures of whoever possesses it. Very well, one of the most common occurences lately in these personalised copies is the appearance of a mysterious anomaly known as "The Wario Apparition". I am here to explain what it is, and why it's there.

''Noone knows where the original build came from, all it was known to employees was that Nintendo wanted to use it to create the first, big 3D platformer. The jump from 2D to 3D was incredibly hard for the company, and thus, they decided, that their first game would not be shown to test audiences would not be a Mario game, but a Wario game, instead, as they feared that if the game failed, Mario would be associated with its failure, essentially ruining his image. And so, Nintendo decided to make Wario Land 64, marketing it as a sequel to 1994's Wario Land.''

The game would function as any Wario Land game would: it would have shoulder bash, harsh movements, and of course, the ability to gather power-ups via being damaged by enemies.

''One of these enemies was the Ghost Spider, that, after the game proved to be successful and was changed from a Wario game to a Mario game, was reused in Wario Land 2 (1998) and onwards. If Wario clashed with the Ghost Spider, he would turn into a ghost, his body would dissolve and his head would start growing drastically, with that, he would be able to essentially roam the level as an invincible giant head, eating any enemy who dared to cross his path. Sound familiar? There was also a multiplayer mode planned, one where two players could duke it out in an arena using certain power-ups, including the Ghost Spider.''

''As mentioned before, after the game proved to be successful to test audiences, most content was reskinned as removed to make it look more like a Mario game. However, the A.I kept most of the content from Wario Land 64, using it to personalise the game. Soon, in Space World' 95, the game would be revealed, and played. The devs began getting paranoid, trying to find a way to remove all of the Wario Land 64 content from it, and all of the content before it was even owned by Nintendo. And so, the devs found a way to conjoin every single bit of content they didn't want and throw it out. And it mashed together, unknowingly, they created the Wario Apparition. The reason it causes hallucinations and stroke-like effects on the players who see it is largely unknown as of now, but it is theorized that it has something to do with whatever the game was supposed to be before it was even inherited by Nintendo. All that is known is that the Apparition's inner behaviours are practically an enigma, it is an incredibly unpredictable being, and if it contains whatever cursed content from the game before it was sold to Nintendo it... Let's just say that wouldn't be favorable at all"''

Guide Book
The 'Story' section of the game's guide book was shared, after someone was sent an email with random gibberish and a single image (Shown below), that is speculated to be the game's guidebook: