I just notice something. The Ballos sprite in Halloween it look a lot to Astaroth (the final boss of Ghost 'N Goblins)
Here's Ballos:
(...)
I can't put a image of Astaroth so simple google ''astaroth ghost n goblins''
According to the Cutting Room Floor, there's unused stuff. The most interesing (for me), is a unused form of Balrog. Here is the page: http://tcrf.net/Cave_Story