Sega has made a good practice of bringing its classic Sonic the Hedgehog titles for the Genesis to as many platforms as possible over the past decade, and while you can play the prime Sonic trilogy (& Knuckles) on the Nintendo DS, there's a piece of key Sonic history missing from that platform. The premiere Sega CD title Sonic CD could've ended up on the DS as part of a project from Simon Thomley, and if that name sounds familiar it's because he went on to bigger things with the recently released Sonic Mania. Back in 2009, Thomley was contracted to work on a pitch to Sega regarding Sonic CD for DS, and though the project never materialized in any official capacity, the DS proof of concept is out there and freely downloadable from Thomley's company, Headcannon. Want to play a piece of the Sonic port that never was? Thomley explains:
Primarily, I was to dissect and explain the Palmtree Panic Zone boss, which is pretty complex in design by comparison to how effortlessly it can be beaten, so that he could reproduce it flawlessly. During this time, I personally reconstructed the boss myself using my existing port of Sonic 1 to GBA/DS to make certain that I was understanding it correctly. At the same time, I also had a personal interest in Sonic CD's Special Stage, which I had also been dissecting, and continued my work with it in order to both provide assistence (though not direct) with Christian's Sonic CD build, and to attempt my own with the DS.
As both projects continued, I saw merit in the idea of pitching the concurrent development of a DS version of the game, which would require such a low-level remake as mine due to the fact that the DS wasn't powerful enough to support a scripting-based game engine like Retro Engine. Given that, I set out to make a complete POC, which was taken pretty much to completion.
Unfortunately, once Christian's version was formally accepted and he was under contract, I would no longer be able to associate with the project and was therefore without a means to get the concept off the ground. I had no other contacts, and with this being Christian's first project with the company, he understandably couldn't take the sort of risk that would have been involved in attempting to take care of it himself.
You can run this software on a real Nintendo DS using a flash cart or in a DS emulator. Be sure to read and follow the instructions. Sonic, Tails, and Knuckles are all playable in Palmtree Panic Zone Acts 1-3 (in all three time periods) which is especially interesting since Knuckles did not make the playable cut in the eventual conversion of Sonic CD that was released in 2011 for other platforms. I love to see developers release their cutting room floor scraps for fans to explore and wish more would do it, although I understand why they don't. This is just one of several similar projects that Thomley has released and I encourage you to check them all out.