The scene from Adventures of Mark Twain when they meet Satan is pretty creepy.
http://www.youtube.com/watch?v=2OsYCFOaCbg
Was gonna post this. Terrifying.
"Life itself is only a vision. A dream. Nothing exists, save empty space and you. And you... are but a thought. "
Well, shit, Satan, this is a pretty damn depressing kids film.
Seriously though the design of Satan in that is a stroke of genius.