Koopa

Koopas are common enemies in the Super Mario Bros. series. They are yellow turtles with (usually) green shells. They will walk around aimlessly, but when attacked will drop their shell. They will then run away. Attacking the Koopa again will make them drop a blue coin. There is a special Koopa known as Koopa the Quick that can be found in Bob-omb Battlefield and Tiny-Huge Island, and can be raced for a star.

Koopa shells can be jumped on and then surfed on until they hit a wall, which will cause it to despawn. The shells can kill most enemies upon contact, similarly to the Metal Cap.

Enemy Discoloration
Main Article: Enemy Discoloring

Enemy Discoloring can create Red Koopas. These Koopas drop shells that can be thrown or kicked as opposed to ridden on. This is more similar to Koopa shell functions in other Mario games. The shell-less Koopa will also chase mario instead of run away. Notably, these behaviors can be found unused in the files of Super Mario 64 DS.

Yellow Cap Switch
Main Article: Yellow Cap Switch

The Yellow Cap Switch is an unused cap switch found in the files of Super Mario 64. It is automatically pressed upon being spawned. It is also associated with transparent yellow boxes that, when activated, can spawn Koopa shells.

Red Koopa (DS)
In the files of Super Mario 64 DS, there is an unused Red Koopa. They run towards the player instead of running away from them, but doesn't hurt the player, likely due to being unfinished. When removed from it's shell, it will chase after it. It still drops a blue coin upon defeat. The shell can then be kicked around, which will shoot in one straight direction before hitting a wall, disintegrating into smoke. It cannot be ridden or picked up. If Yoshi eats this shell, it can spit fire, just like Super Mario World. In some personalized copies, they can be found in game, sometimes even fully functional.