Hammer Bros.

In Super Mario 64, there is a waterfall next to Peach's Castle. It is rumored that it is possible to go behind the waterfall and find a secret 120 star door. The door would lead to a secret boss fight against the Hammer Brothers, similar to Super Mario Bros 3. This rumor was popular around the same time the L is Real 2401 rumor became popular, although it never seems to be discussed anymore. Traces of this rumor's existence can still be found, in a video by DidYouKnowGaming, mistaking the rumor for being a Hammer Suit power-up instead, and also in the Super Mario 64 Neoseeker forums. This rumor may have been due to the Personalization A.I creating a new boss fight, like how the L is Real rumor could be related back to the A.I calling in beta assets for Luigi. This rumor is presumed to be started some time in the mid-2000s, and died off around the early 2010s, around 2010 or 2011, unlike the L is Real rumor, which is still popular to this day, especially now due to the leaks.

Accessing the Boss Fight
Getting behind the waterfall is tricky enough, but it can be done either by doing some well-timed movements off the top of Peach's Castle, or by using a moon jump code. If it does not work, that means that you do not have the Hammer Bros. in your copy, or your copy is not affected by the A.I. If it works, you will be greated by a stone hallway, and a 120 star door at the end of it. 2 yellow ! Blocks are next to the door, which give you some coins and a 1-Up. If you have all 120 stars, the door should open. The boss arena is themed after the Cavern of the Metal Cap/Behind the Waterfall, the stage where you find the green Switch, and consists of a circular floor, and water circling it.

Hammer Bros. Boss
The Hammer Bros. fight consists of 2 Hammer Brothers, with their design looking similar to their Mario Party appearance, but with a body shape more similar to Koopa the Quick. The fight starts with some entry dialogue. "Look! It's Mario! He thinks he can take on some of Bowser's top Koopas. What a silly statement! C'mon. Let's show this plumber who the real elites are." One will come for Mario, and the other will stay behind and throw hammers. Mario must jump on both of their heads 3 times to win. Defeating the one chasing you makes the other one stop throwing hammers and also come after you. Defeating the stationary one makes the other Hammer Brother faster. The noises they make when defeated are a lower-pitched Koopa sound. Defeating both of them gives you a Power Star, and some more dialogue. "Augh! That stupid plumber beat us! What is Bowser going to do with us now? We'll get back at you for this plumber!"

Theorising
It is unknown how the assets for the Hammer Bros. were put into game, and where the A.I got them from, although it can be speculated that an unused Hammer Bro enemy could of been planned for the retail Super Mario 64. Why they are placed behind the waterfall is unknown, too. The fact that these Hammer Bros. appear only as a boss is the most interesting part however, which could mean that the unused Hammer Bro could of been an unused boss.