The original Sonic the Hedgehog had some of the most bizarre pre-production stuff I've seen in a while. The makers of the game were bouncing around a lot of ideas about the game in the early days of it's development and it shows with a lot of artwork such as Sonic the Hedgehog as a rabbit, and Sonic apparently fighting demons with Robotnik dressed as a bumblebee.
Also, there's a lot of graphics that were apparently deleted from the final version of the game, these cannot be found on any ROMs but could be seen in magazines and really old game commercials. A good example is this french Sonic 1 commercial which shows Robotnik's chainball rotating and also the spring "stetch" when Sonic runs into them.
https://www.youtube.com/watch?v=1hKMVkb0k1Y And this advertisement which shows some of the unused Sonic the Hedgehog zones like Sparkling Zone and Clock Work Zone. Sparkling Zone also had different graphics than those used in the final game as well and there were UFOs in the background of Marble Zone, I'm not making this up, search and see for yourself. Sorry if I took up a lot of space, this pre-production stuff is interesting I think, the best place to look for it is old vintage gaming magazines prior to the game's release.