Yoshi Racing (lost Argonaut Nintendo 64 pitch prototype; 1995)
After they successfully released the Super FX chip-enhanced game Star Fox in collaboration with Nintendo for the Super Nintendo Entertainment System in 1993, British game developer Argonaut Games came up with the idea for a virtually new type of game: a 3D platformer.
They threw together a prototype game of this genre featuring Nintendo's character Yoshi.
When they showed Nintendo the game, Nintendo rejected it, not willing to take the risk of entering an entirely new field with an outside developer.
This was the final blow to the two companies' relationship, as Nintendo soon released their own 3D platformer for the new Nintendo 64 console, Super Mario 64, which ended up using several of the ideas Argonaut implemented in their Yoshi prototype. Shigeru Miyamoto later apologised to them.
This prototype has never surfaced, and no one outside the two companies has seen any material related to it.