It isn't a portmanteau, it's a classic Greek word construction (an ordinary expression of classic Greek morphology). "Morph" generally just means "shape" as a Greek root so "xenomorph" is more accurately "alien-shaped". Which gets back to it being a very generic term said in a fancy way (like "we don't even know if it is an alien, we just know it is alien-shaped"), like much technical and scientific jargon will do when it goes to constructing Greek words to describe something ("gynomorph" => "woman-shaped"; to call to a different sci-fi horror Species).