It should be clear by now that I really like weird and alien critters. So when I see something that is some form of "hypothetical" exploration of what life might be like on other planets or in the future, I pretty much have to take a look. (And if you're wondering what brings it on, I've been looking around in preparation for the logical sequel to this article.)
Take this ancestral counterpart to the speculative documentary, made by Disney in 1957(!):
I cannot emphasize how much I love this footage. Since I found it on YouTube the other day, I've watched it at least half a dozen times. (I may have seen it once years ago, but I can't be sure.) It's just so imaginative, so rich, so weird. Sure, it's total nonsense, but speculation on what Mars could be like if it had life could easily be applied to virtually any other planet, real or imaginary.
Also, an awful lot of those critters have a weird, inexplicable ability to eat things merely by wrapping around them, don't they? (My favorite example of that is the flying magnifying glass creature; where are its guts if its central body is the lens?)
I'm more familiar with the works of Dougal Dixon, who has pretty much made a name doing this sort of thing. Perhaps his biggest and most ambitious project was The Future Is Wild!, which was about future ecosystems if there weren't humans to worry about.
Basically, he asked the simple question of "what could these animals turn into?" And his answers were often unconventional. (In this case, he had help-it was obviously a reasonably high-budget project. It wasn't the first time he was a driving force behind a project made for television, though-1997's little-known Natural History of an Alien appears to have been heavily driven by Dixon's work.)
Of course, one can take the ideas further, and few take them further than Wayne Barlowe, one of the world's most famous science fiction illustrators. To get an idea of just what kind of guidelines he had for his work for Expedition, the book upon which the Alien Planet "specumentary" was based, he remarked that he decided early on that there would be no eyes or ears.
I need to find my Barlowe's Guide, darnit.
There are other, lesser-known efforts at specumentaries, as well, such as this one, known in the US as Extraterrestrial.
Great stuff, if you're ever short for ideas, or even if you just dig that sort of thing.