the maker of the gif (since its not on youtube) shows that through editing the game files, you can spawn an unused "betty beet" stage of the boss pack! it is really broken and was supposed to throw mini beets and was cut for unknown reasons. (probably since the beets would be thrown randomly making it harder to avoid than any other phase) it also was gonna appear before weepy the onion for more unused bosses see this: https://www.youtube.com/watch?v=bCb5fEyVMpA
full credit to cuphead and their really broken beet boss phase