Meowth's Party (Rumored GameCube game)
Meowth's Party was a tech demo shown at Nintendo Spaceworld 2000 that was created by the same team who worked on Pokemon Stadium to show off the GameCube's graphical capabilities. The demo features Meowth singing and dancing on the stage, where he soon gains a crowd of various Pokemon to watch his concert.
There have been multiple rumors surrounding the tech demo about it originally being a concept for a game. It may have been "cancelled" due to the developers feeling like there wasn't enough material to make a game based on the demo's concept. However, the concept was ultimately reused in the GameCube game Pokemon Channel. It is still unknown if this would have ever been a video game and if any development for it was done, and as for the original tech demo, it has not surfaced anywhere online and hasn't been shown off since Spaceworld 2000.