From my understanding Engineers originally created them. David created the Xenomorph by reverse engineering from the Black Goo with the Deacon in mind. Basically everything is just a variation of the classic Xenomorph as we know it.
If you look at the text from David from his experiments it indicates he is reverse engineering with an end goal in mind which is to recreate something that the Engineers already had(which the mural shows). So he already knows roughly what it's supposed to be in it's final form(i assume from the previous Engineers creations)
"This mutated specimen appears to be nearing perfection. Investigation of this series of mutations show that the final specimen is within reach. The development of the proboscis which appears as an elongated appendage from the head of the creature seems to provide the link missed in previous specimens."