A 1993 issue of Sonic the Poster Mag contains images of what people have called an Early Sonic the Hedgehog cartoon. These images are part of an article which contains info about the two upcoming Sonic cartoons, Adventures of Sonic the Hedgehog and Sonic the Hedgehog. The images could represent either cartoon but the serious tone of the action in the images and the inclusion of a Freedom Team, which sounds like a early name for the Freedom Fighters, imply it was an early version of Sonic the Hedgehog. It is also possible that the images represent a totally different cartoon altogether.
These images feature the Princess Acorn character.
In this image several characters are shown Johnny Lightfoot (rabbit), Tux (penguin), Princess Acorn (probably a squirrel), Sonic, Joe Sushi (walrus), Chirps (chicken), Porker Lewis (pig), and Flicky (bird). Tails isn't shown, so the concept art may have been created before Sonic the Hedgehog 2 was even released.
The characters seem to be based on the animals rescued by Sonic in the first game.
The characters Sally, Bunnie, and Rotor have similarities to Princess Acorn, Johnny Lightfoot, and Joe Sushi as in they are based on the same animals. The Porker Lewis in the image has a different design than he does in Fleetway's Sonic the Comic.
Robotniks design is different from any of the cartoons. He has black fingernails, a white mustache, and a green tongue.
This conveyor belt roboticizer is totally different from the cartoon version, which was a glass capsule.
One character is shown which doesn't have a corresponding animal in the games, a wolf. It's not clear if it was to be a supporting character or just a random character thrown in the picture. The wolf concept was used for the character Antoine in the SatAM.
This sales card shows the differently designed Robotnik and Princess Acorn. It also has a mention of Sonic the Hedgehog and Adventures of Sonic the Hedgehog on the back of the card, meaning this card represented at least one of them. This means one of the cartoons was meant to have this Robotnik design.