[Alamo-Girl]

Thank you so much for the further explanation!

Indeed, Hollywood monsters seem to be mostly derived from known earthly body plans. IMHO, the aliens in the Star Trek series were more diverse - creatures without structure, micro and macro creatures, collectives, etc.

[PatrickHenry]

It must be difficult for a truly new body plan to get itself established. It probably won't spring fully-formed from some existing species (way too many mutations have to click into place all at once), so it would have to evolve, step by step, from some far less complicated organism. I suspect the window of opportunity has closed on that. The available niches are somewhat crowded now.

Loads of new insects are always turning up in the rain forests, but they're still insects. No doubt the oceans still have surprises which await us. However, on land ... it would be quite a trick for a whole new line of evolution to get itself started. It would be foolish to rule it out, but the most favorable time for such developments would seem to be after a mass extinction event. That's how it looks to me.