After the release of Super Smash Bros. Brawl, people discovered that there were data files for seven characters (colloquially referred to as "The Forbidden Seven") that were cut during the game's development. The filenames are "mewtwo", "roy", "dr_mario", "toonzelda", "toonsheik", "dixie" (the director of the game, Masahiro Sakurai, has confirmed that Dixie Kong was going to be part of Diddy Kong's moveset), and "pramai" (which is commonly theorized to have been Plusle and Minun from Pokemon).
I've recently wondered if there might exist a beta build of the game with any of these characters. Mewtwo and Roy have the most data out of the seven characters, and both were in Super Smash Bros. Melee, so it's fair to assume that both were planned to be in the game from the beginning and were cut late in development. This could imply that those characters were once at least close to being in a playable state.