Here's a clip from a low budget American horror movie that Go Nagai did the concept art for. I'd like to think that Go's final design of the monster made it into the finished film.

Any pictures of the concept art?

Found 'em! The monster was supposed to morph into more horrifying forms, which would've made it cooler!

Oh, I've read about those! They were sold in an auction apparently.

http://tomfranckart.wordpress.com/2011/11/03/the-five-mysterious-paintings-of-go-nagai/

Yep! That's them! So now you know where they're from.