Let’s start by raising the major spoiler flags. Unless you don’t care at all about the plot or seeing how the final boss of the game looks like or is named, turn back right now. Everyone else, let’s press on.
The Japanese Sonic site updated yesterday with a slew of new content, primarily with new images from the 3DS version of the game, a few from the console version, and some character bios. However, thanks to some sleuthing and confirmation from Sonic Stadium member Solofiveo, changing numbers in the image file names bring up this little image.
Hard to say what it can be, since it looks like a monster of the week.
Of note, when changing character image numbers, this particular one being found at “sonic.sega.jp/SonicGenerations/blue/character/css/contsnts-character/slide_04.png”, you get the following:
01. Sonic, 02. Tails, 03. Eggman, 04. Time Eater, and 05. Metal Sonic. Two more slots, currently 404ing, exist afterward, belonging to Shadow and Silver respectively.
While both Sonic and Tails feature their classic and modern forms together, Dr. Eggman is by himself with no sign of a Classic Eggman render.