Rabbit Meadow

Rabbit Meadow is an anomalous secret level found in some personalized copies of Super Mario 64 DS. How it's found is inconsistent, but it usually requires finding every rabbit in the game, even the Purple Rabbit if it appears, and obtaining all 150 stars, though this is less common of a requirement. The level is described as a large meadow filled with rabbits, hence the name, and even has a boss fight against a Rabbit King. The level appears to parallel the Purple Prizes magazine scan's reported method of unlocking Waluigi, potentially being the personalized level those who made the magazine found.

Description
Rabbit Meadow is made up of several sunny hills, and multiple rabbit NPCs that one can reportedly interact with that talk similarly to Toads. However, they might act normally, running away from the player unless caught, which either makes them drop keys, most of the time useless as you've already unlocked every minigame, or makes them enter their dialogue mode. There are several trees scattered around, especially at the level's perimeter. Somewhere in the level, a castle can appear, that reportedly houses the Rabbit King. The castle itself is usually designed similarly to Princess Peach's Castle, but with slightly grayer bricks and a vastly different shape. This area itself is a sub-area, similar in design to Whomp's Fortress Interior, usually leading to the top of the castle where the Rabbit King resides.

Missions
There are 3 missions most commonly reported in Rabbit Meadow: A race against Koopa the Quick, a Star Switch mission, and a boss fight against the Rabbit King themself. Notably, upon beating Koopa the Quick, many claim they say the player is the "fastest footracer in the land." Other supposed missions include catching a specific rabbit in the meadow, which is occasionally the Purple Rabbit, collecting 8 red coins, collecting 5 Silver Stars, or unlocking a cage inside of the castle. The final mission reportedly only appears after beating the Rabbit King, and can even be a requirement to unlock Waluigi, as many claim he is held captive in this cage. However, some also say that the only mission is fighting the Rabbit King, similarly to other levels tied to unlocking characters.

Rabbit King
The Rabbit King's arena is circular, like most other bosses, taking place on top of the castle. They jump around, leaving shockwaves similarly to Bowser, attempt to kick the player, and charge at them. To defeat them, the player has to grab them by the leg mid-kick and throw them off of the castle. This is usually repeated 3 to 5 times before they are defeated, and reportedly can drop the key to unlock Waluigi, making this another method of unlocking him.

Theorizing
The 3 main missions of this level appear to mirror the instructions on how to unlock Waluigi in the Purple Prizes magazine. First, it requires the player to obtain all 150 stars, which is a supposed requirement to find Rabbit Meadow, and then it tells the player to become the fastest footracer in the land. Koopa the Quick races the player and can even call the player said title after being defeated. The "secret switch" could be the Star Switch, and the Rabbit King boss fight matches up with Purple Prizes, as it reportedly drops Waluigi's key. Notably, some say the castle can only be entered after beating the Star Switch mission, which matches up with the cage Waluigi can reportedly be found inside, as the switch would "reveal his door."