Sonic the Hedgehog CD, or simply Sonic CD, is a platform game in the Sonic the Hedgehog series, released for the Sega Mega-CD in Japan on September 23, 1993, in Europe in October 1993, and finally for the Sega CD in North America on November 19, 1993. After the release of Sonic the Hedgehog, Lead Programmer Yuji Naka had grown dissatisfied with the rigid corporate policies at Sega, so he moved to the United States to work with the Sega Technical Institute. Incidentally, a large number of the original design team of Sonic also left for the U.S., to help instruct the American developers. With half of Sonic Team and two of its most important creators present, the Sega Technical Institute eventually got the job to develop Sonic the Hedgehog 2.
Meanwhile in Japan, Sonic CD (or at this point, “CD Sonic” as it was first known) was handled by a separate development team, headed by Sonic creator Naoto Ohshima. Initially, as revealed in interviews and magazine clippings, Sonic CD, and Sonic the Hedgehog 2 for the Mega Drive/Genesis, Master System and Game Gear were supposed to be the same game. However, during development, Sonic CD evolved into a vastly different type of game. [Info from Wikipedia]
An early version of one of the bosses in the game had pinchers instead of bumpers on its hands. In the beta version of the Special Stages, the background had a large mountainside and planet instead of the usual dark space. The UFO was originally a baloon which Sonic would have to pop.Various unused sprites can also be found in the game’s code. Rumors also had it that Tails was going to be in the game but it was later removed. A couple of removed Levels were know as R2 and Final Fever.
It seems that various Sonic CD Betas were leaked online, as the ones labeled 510, 712 and 920. The Sonic CD 510 Beta has many differences that were seen in early screens, with changes in the graphic details, removed level sections and unused monitors (S and Gem monitors that transform Sonic into Super Sonic and a Timer monitor that would have stopped or slow down the time). As we can read at Sonic Retro:
Sonic the Hedgehog CD prototype 510, referred to as CD Sonic the Hedgehog on the title screen, is among the first prototypes of the game Sonic the Hedgehog CD for Sega CD. This prototype was discovered by a group of fans called Sega Extreme in the summer of 2000. The game contains all the stages of the final game, but at first sight the player can only highlight the first three: Palmtree Panic, Collision Chaos and Tidal Tempest. The rest can be viewed through the Level Select. Secrets also are in the game. Using the Sound Test menu and entering a Specific Code for each one will reveal some secret artwork. One is a cute Sonic with Japanese text that reads “You are cool”. Another one is a Scene where Sonic and Metal Sonic rap and Robotnik as the DJ. Another one is a epic tribute to badman. The last one is so creepy im not even posting. It appears both by using the sound test AND running a pirated version of Sonic CD. Its Sonic with a scary face with no pupils and looking mad with Japanese text that reads something like “Fun is never ending with Sega – Manjan (which can be translated to Devil or Demon” So you probably know why now.
For more info you can check the SoST archive and X-Cult!