The Proto-Xenomorph made its first appearance in Prometheus, at the end when it burst out of the dying Engineer's chest. It was formed from an embryo the Trilobite impregnated the Engineer with. Note how the birth of this Xenomorph is different to a majority of other Xenomorph births, as it appears to be a very basic yet humanoid form of the usual Xenomorph. The fact it spawns almost fully developed makes it quite unique amongst other Xenomorphs seen in the series; though the dog alien chestburster had arms and legs upon bursting. It still might be just a chestburster as it is small compared to its host but not to humans which is why it looks like it is fully formed, however; it is possible that this is still a small phase of growth, and it will grow to be gigantic in size, much larger than the Trilobite and Engineer, as with the standard Xemomorph's growth cycle proportions. Unlike normal Xenomorph chestbursters, which "punch" their way out using the front or top of their heads, this creature used the sharp back end of its head to tear a way out. Also unlike normal Xenomorphs, it does not have an inner jaw and instead has a human/Engineer tooth structure and gums. Its "inner jaw" is merely an extra jaw that protrudes a short distance but is much larger and more unwieldy than a Xenomorph's inner jaw. Whether this "inner jaw" can be used as weapon is unknown. This version of the Xenomorph will evolve over time into the regular version, which the Engineers(Space Jockeys) use as a weapon of mass destruction.