A look at various Metroid titles over the years has brought us to Metroid Prime: Hunters, which took its sweet time getting out on the DS. With that said, it looks like there was a ton of content that just never made it out into the final release. Enemies, design choices and event animations that were cut. You can check out all those goodies at the link below.
At the link, you'll also find some very interesting Metroid art that never made it into any game we've ever seen. This artwork comes from a former Nintendo Software Technology Studio employee, and while it doesn't mean anything was in development, it's certainly interesting to see what might have been.