alien creation in movies

I'm not picking faults with movies or the designers here, just what I have been thinking.

All movie creatures never truly look like an alien may look. take Geiger's alien as an example don't get me wrong I love the movies and the design and there's no real way of doing this, but when designing a monster or alien we only have animals of our own planet to refer to in that design and you can easily see it in all movie monsters such as the xenomorph from the movie alien it is a cross between a scorpion (its tail and exoskeleton, an insect in the way it lives in its hive hierarchy, and a human because of its being able to stand on its hind legs or bipedal movement. in other words in order to make a 'true alien' we'd have to have actually found aliens and since we haven't it is impossible to know what it could look like in reality. It would have to be something unlike anything on this planet, which again is impossible because in the design process animals from this planet would be an influence in the process and imagination. As it goes we can only be influenced by the animals we have seen in books, documentaries etc. For all we know at this moment alien life may only be bacterial in nature.