The classic ones are also based on existing art. Like, the last two rows of Classic Sonic art are either ones I've seen...somewhere (the one looking forward and crouching may or may not be taken from a pose in the Sonic CD intro?), while others are absolutely based on the old original black and white sketches, like that one where he's looking with his hand above his eyes. You can tell because they're significantly more noodly. Also, the one in row four right in the middle is based on an Ohshima drawing but they gave him straight teeth instead of the pointy ones.