What is the difference between a Monster of the Medieval Fantasy variety and an Alien of the Sci-Fi/Space Adventure tradition?
Numerous blogs before me have addressed this issue and I'm sure many will after today but I was thinking about it and I have a blog so I figured I'd give my two cents on the subject.
Honestly, I see them as fundamentally different from an atmosphere and purpose angle, at least as it relates to RPGs.
A Monster is a creature of myth and folklore that represents humanity's fears, brings to light a trait in the human character or otherwise illustrates a lesson or failing in our collective experience. It is this description which makes me so fascinated by medieval bestiaries and less than enamoured of the D&D versions of monsters. Unlike the lesson in humility of yore that the Manticore represented, the D&D Manticore is a big thing with sharp teeth that you kill for stuff. The story of the Minotaur is a story of a unique creature resulting from a most unique situation that mirrored political and social elements between Greece and Crete. In D&D its a big thing with sharp horns that you kill for stuff.
I digress...the thing about monsters is, for them to be monstrous in my opinion, you can't know too much about them. They shouldn't, in a manner of speaking, make too much sense. Descriptions of monsters with No. Appearing, Alignment, % in Lair and the like do the Monster a disservice. Monsters are weird, mysterious, menacing and information on them differs to region to region and storyteller to storyteller.
As awesome as the old Dragon Magazine 'Ecology of...' articles were I eventually started to really dislike them. You want to know the Ecology of the Tarasque? Fine, here it is; Twenty minutes after a Tarasque comes into your region there is no ecology. Everything is dead. This thing is a ~#$king Monster! A Monster doesn't belong in your local ecology. Its an X-Factor, an afront to or freak of nature. Monsters are supernatural speed bumps on the road to evolution that Darwin missed on his last drive by.
Aliens on the other hand are a whole other...er...beast. Aliens need to make sense. They need to fit. They need to be a part of the package that includes an exoplanet's temperature, weather conditions and terrain. If a 50 ft long serpentine Dragon weighing in excess of 5 tons (about the height of a T-Rex but lighter in my mind) moves stealthily through a misty forest no one shouts "How? Where does it find enough food to support its size? How can it fly with that size wingspan and a weight of 5 tons?". Its a Dragon. Its there. Now run for your lives! If that were an Alien though it would need a darn good reason to be where it is. This makes Aliens much harder to design than Monsters but to me much more rewarding.
If an Alien works, it not only becomes a memorable element of an adventure but it makes the whole planet the PCs are on seem more real. Every time I've used a really good Alien my players are all, "Whoah now I get it. Its perfectly adapted to swim through this muck/climb this mountain/detect us without visual senses, etc. We need to change tactics!" They remember the planets and their unusual conditions as well as their encounter because these elements are intrinsically related.
An Alien is all about how it fits in to the world it lives in. A Monster, at least to me, is all about how it doesn't.