Koopa Troopa Plains

Koopa Troopa Plains 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 Bob-omb Battlefield as the very first course of the game. Described as large grassy area, it shares many similarities to Bob-omb Battlefield, but more themed towards the Koopas, with another race with Koopa the Quick being reported here. The theme of the course is a remix of the original Super Mario Bros. overworld theme, which can sometimes be reported as the theme of Pleasant, Pleasant Falls as well.

Description/Geography
Koopa Troopa Plains is described as a large vast area, with mostly flat land, and a couple of hilly areas around the course, filled with trees and a small barn near the right edge. Enemies reported to be found here include Koopas, Goombas, Bob-ombs, and Chuckyas.

The course begins on a small pile of dirt leading to a gravel path which the player can take to go down multiple paths, including a path to the barn, a path to a hill with a giant Koopa shell, and a final path leading to a box with the Wing Cap, which can be taken to reach a large platform with a box containing a star. Another path will also take the player to the main boss fight of the course with Hookbill.

Missions
The most commonly reported missions are the red coins star, with the red coins spread around the plains, the aforementioned star requiring the Wing Cap, along with another star that can be won from Koopa the Quick by racing him. The final most commonly reported mission is a unique boss fight with Hookbill the Koopa from Super Mario World 2: Yoshi's Island. The following text was found for Hookbill before battle: "You've finally made it, huh? I'm no ordinary Koopa, I am the grandest of all the Koopas, you can call me Hookbill. So Mario, you think you can just waltz in here and expect to get out with Lord Bowser's well earned stars? I don't think so buddy! Come on, time for me to rock your world!"

Hookbill is an interesting boss fight, while he looks like an overall smaller version of Bowser, his attacks mostly consist of Hookbill charging you, where you have to kick him to stop him in his tracks, then kick him two more times to knock him to the ground, you then must ground pound him to make him take damage. Repeat this twice more to defeat Hookbill. The following text was reported after Hookbill was defeated:

"Defeated by such a tiny human like you? How did that happen? Don't feel so proud of yourself for beating me, I'm only one of Lord Bowser's many minions set out to defeat you. Since you've won, it looks like I must surrender my star over to you. I still can't believe that I lost to you out of all people."

Theorizing
There are several theories of why Koopa Troopa Plains was removed from the game. One rumor believes that if the course was to be on the 4th floor, it would be way too easy after the game. Others believe the course was removed because the developers liked Bob-omb Battlefield more, or the level was too similar to Bob-omb Battlefield. Still others claim it was because of the Hookbill boss fight not being able to function properly.

The course would never make it into the game, and while many elements of plain levels were used in other Mario games, Hookbill has never made a mainline appearance in a Mario game since Super Mario Advance 3. Another interesting detail some have noticed is that Hookbill's text was very similar to that of Goomboss in the DS remake released in 2004. Many believe the text was reused for Goomboss from Hookbill.