After unlocking the storyboards, if you look at the last one, you can see that the Lizard was meant to be in the final cutscene.
In the "Race to the Bugle!" Level, during Stan Lee's narration you can see a fiery image drive up the building. Once the level starts and you head out, you will see it again.
The figure is Ghost Rider. And his cameo is exclusive to the Dreamcast version of the game.
Spider-Man uses the same game engine as Tony Hawk's Pro Skater. Spidey also was a hidden character in THPS2.