I believe I've seen it, too, but the drawing was in black and white. It was probably on an ad in some magazine or manual. Or one of those flyers that often came packaged with NES games.
Also, that dragon is not actually in the game.
It's probably supposed to be the final boss, but they took liberties with the design. The actual final boss is like a mechanical or skeletal dragon, if my memory is serving me well enough.