I don't want to unnecessarily necro a thread, but I'm pretty sure this doesn't warrant a new post.
The most oft repeated Trope of alien lore is the bipedal humanoid, and I think it would be a brilliant thing if there were more games that challenged the status quo on this one. So I guess in this post I'm aiming to think outside the box a little.
Firstly, if there is not a common evolutionary lineage within the past few million years then there is no reason to assume aliens would be bipedal, the vast majority of creatures on earth are not bipedal. The majority are quadrupeds, but even that is likely a shared evolutionary trait inherited from previous successful organisms and requiring a significant beneficial mutation to overcome. In short, bipeds are likely to be rare, some examples of alternate forms on earth might include:
Millipedes
Octopi
Squid
Jellyfish
Snakes
Fungi
Swarm
Fish/Shark/Dolphin
And the more familiar intelligent forms:
Pig/Elephant
Cat/Dog
Bird
Ape
Other forms not found on earth might be:
Cloud
Energy ball
Shape changing
Invisible
Any of these forms could conceivably give rise to intelligence and tool manipulation, and should be considered as likely as a biped.
It is also worth considering that if Alien beings were composed of substances lighter than air then it might be possible for them to float naturally in our atmosphere, or perhaps they have developed some technology to help them move. Following along those lines, realistically if we were to visit another planet we would be unlikely to be in our raw forms even if the atmosphere were breathable. These points would lead us to believe that an alien on earth would most likely be inside some kind of shell, both for protection and for life support. Possible forms might include:
Quadruped
Wheeled
Tracked
Hovering
burrowing
Flying
Teleporting
For these reasons, and others, I would like to submit the following Ideas:
rocket propelled shark (flies, fast, strong against physical bullets, weak against fire, uses melee attack)
Diamond Covered Octopus (slow, heavily armored, uses multiple weapons )
Teleporting Armored Energy Ball (heavily armored, can teleport through walls, weak to energy weapons and explosives).
Shape Changing Cloud (immune to physical weapons, can move through windows, can take on the form of any other creature, weak to fire, explosions and energy weapons)
Burrowing suicide torpedoes (very hard to target, moves under walls, explodes on impact)
Hovering Jellyfish (paralyzing melee attack, can move over debris, weak to physical weapons)
Snake.
