Prefacing this comment with: I do not give a fiddler's fart who created the xenomorph. Literally do not care if the creator is David.
That being said, I'm finding it kind of hilarious that people believe David "created" the xenomorph full stop. How could he have? He must have recreated it. There's no other explanation with what we now know. Saying he's some kind of sole originator would be like saying a guy who used a computer to print a piece of paper is Bill Gates. Without the actual creators, the Engineers, there would be no black goo. No creature at the end of Prometheus. No purpose to the black goo. No threat that they were running from, no murals, no biomechanical aspect to everything.
He'd literally have nothing to go on. He stood on the shoulders of giants. He replicated, he didn't create.
In fact, I thought that was one of the cleverer themes of the film. He wants to be an all powerful creator, but he isn't. In the end, he's just like Walter. Mimicking, pretending, trying hard but not really succeeding.