I don't think it's too much to ask that I be allowed to Profile a fictional character (or in this case, species), especially not when they've had the impact on modern cinema that Xenomorphs have had. It will definitely make for a shorter Profile, as I can't fill space talking about the roles they like to take (extinction level events) or what they do in their free time (show tunes; Spaceballs joke alert!). I can, however, pontificate on their portrayals so far, and potential going forward. It probably goes without saying, but there will be spoilers for movies featuring the Xenomorphs in the next couple paragraphs.
To say the Xenomorphs have a terrifying countenance is an understatement, to say nothing of their entire body, but you already knew that. Anything designed by H.R. Giger is going to be at the very least discomforting, and at worst the sort of thing that would make the Devil sleep with a nightlight. Seriously, look at that picture to the right. That is not the face of a man with normal thoughts. Limited by the technology of 1979, Giger and Carlo Rambaldi combined to create the scariest latex suit of all time that even included, in a move that should surprise absolutely nobody, part of a human skull for the head. While this obviously made it very hominid in appearance, even given the actor's 7'2" slender frame, the newer movies have used CGI or animatronics to continue to make them less like a human (save for one ill-advised return towards human that should never be mentioned again), giving them a digitigrade stance (standing on their toes; more birdlike), two fingers and two opposing thumbs on each hand, and making their tails more substantial.
Prometheus set up an interesting genealogy with the Xenomorphs. Namely that the "Deacon" seen at the end of that film might be the progenitor of the eggs encountered in Alien (until the aforementioned trainwreck of a script moved it to a different planet designation). This would mean that the Xenomorphs who are still so formidable, have actually been "watered down" twice by human genetics, which opens us up to all kinds of interesting story options. If they've been designed as ultimate killing machines by the Engineers, isn't it conceivable they can integrate their different iterations (i.e. the quadrupedal Xenomorph born of a dog) into a combination for their next generation that would be stronger because of its genetic diversity? Exposed to a planet like Earth with plenty of species instead of just humanity would give them ample amounts of genetics to combine. Now that could make for one heck of a jumping off point for a movie...