I've never seen The Thing, but a couple of weeks ago I read the original short story by sci-fi legend John W. Campbell, called "Who Goes There?". The lovely red-eyed blue critter you see to your left is the Thing. Great name, eh? I scanned the image from my copy of Barlowe's Guide to ExtraTerrestrials, which is an old artbook done up as a field guide to alien species. Pretty cool, though most of the species are from stories that are fairly obscure nearly thirty years later.
I'd love it if someone put out a similar guide- but of course, there are now thousands upon thousands of stories to glean aliens from.
What species would you have in there? I nominate (for starters, anyways) the qheuen, from David Brin's Uplift Storm trilogy. Qheuens are a crab-like, five-legged race who speak from leg-vents and see from a 360-degree vision strip on their heads. And hoons and g'keks from the same series would be very cool to see too.
And anything from Dan Simmons' Ilium/Olympos- Setebos, Caliban and voynix. Also the shrike from his other masterpiece, the Hyperion Cantos.