Hopper

Hopper is an unused enemy for the game Super Mario 64, discovered in the files for the July 25th 2020 leak (alongside Motos). The enemy appears to be a textureless grasshopper, and it has a single animation for preparing to hop, then hopping.

Hopper would hop around in stages, as evident from its only animation. No behavior code for Hopper seems to exist, so this would be the theoretical behavior for the enemy. This means that the enemy was most likely scrapped shortly after its creation. Interactions between it and Mario are also unknown, given the circumstances above.

Theories/Speculation
Given the name of the enemy and its model's colors, it is most likely that Hopper was replaced with the Money Bag enemy. Money Bag's behavior matches somewhat with what Hopper could have behaved like; Money Bag would hop around and walk for a few seconds after four hops, after which it hops around again.

If Hopper was an early version of Money Bag, this could provide a possible reason for Hopper being scrapped from the game: Hopper has a lot of animation bones, and processing the animation of these bones would be a very expensive usage of resources on the game's hardware. With Money Bag, the game would only have to animate about 3 bones at any given times, improving performance. It is also possible that the design was changed because Hopper's appearance looked too realistic compared to the other characters in-game. The enemy has no textures, so it's unclear how the latter theory may have influenced Hopper getting cut from Super Mario 64.

It is also theorized that Hopper would've been an earlier version of the Crazy Box item, considering that the animation for the Crazy Box is called "hopperjump". This could be a connection between the two entities. It is also possible that judging from this, Hopper would have been a rideable character much like Yoshi.

Trivia

 * Hopper, Motos, and Blargg all were seemed to be scrapped from the final version of Super Mario 64 at the same time.