Hopper

Hopper is an unused enemy believed to be a grasshopper found in the files of Super Mario 64 after the Nintendo Gigaleak. It's model is seemingly unfinished, though notably would be around as small as the Butterflies. It has no associated animations or behavior code.

Theorizing
It is believed that Hopper would have jumped around like a grasshopper, judging by it's name. There is a theory that Hopper was an earlier version of a Money Bag, due to similar behaviors with jumping, as well as a theory it was a beta Crazed Crate due to the jumping animation of the Crazed Crate being called "hopperjump". An extension of the Crazed Crate theory is that the Hopper would have been ridable, similarly to what was intended for Yoshi.

Final Game Manifestation
In some personalized copies of Super Mario 64, a finished version of Hopper can appear somewhere in the game. It's behaviors are inconsistent, with some reports saying it was a harmless animal, with others saying it functioned as an enemy.