Sonic DS (Lost Nintendo DS Tech Demo; 2004)
Upon the release of the Nintendo DS in 2004, SEGA was planning to release a Sonic the Hedgehog game for the new system. At E3 2004, they showed off a tech demo titled Sonic DS. It appeared to be a 3D platformer featuring the titular hedgehog. The level was largely flat, and the game was controlled by quickly moving the stylus to the left and right of the touch screen, making Sonic run, running faster depending on the speed of the stylus. Touching the top of the touch screen caused Sonic to jump, provided any obstacles showed up.
It is unknown if it was a mere tech demo or if SEGA actually intended to develop an entire game based on the concept, but nothing ever came of the demo. Sonic's first appearance on the Nintendo DS ended up being Sonic Rush, a 2D platformer. The demo has never resurfaced, though a fake ROM was distributed via torrent.