Creation of the Xenomorph...

IMO that's a Deacon or a different protoxenomorph in the center.

But... there was possible Xenomorphs on the left and right and also possible facehuggers... but Ridley must have decided retcon them.

This is just my opinion on the matter.

Why do you think anything was ret-conned? David used the Black Goo to experiment and try to re-create the creature the Engineers knew and had depicted in the mural. The movie shows that he has made a close copy, but it is different from the original Engineer-known version.

Probably an Easter egg.

For me David being the creator is still open for debate.

David says "... I found it... I created it...", in that order.

The novelization goes even further when David shows Oram an egg containing a dead facehugger. Claiming that he found the egg but had to kill the facehugger coz it was too aggressive.

David tends to lie through omission.  Moreover he was bragging bout his "works". People will argue that D. lied to Oram to make him "trust" David.  

In the egg-chamber when Oram stares into the opening egg:

D.: "... it's perfectly save (for me)"

The crashed Juggernaut:

D.: "... Part of the payload accidentally deployed when we were landing ... (After i bombed the shit outta them)"

Remember they found black "puddles" in the vicinity of the ship. In the book they even find smashed urns/vases inside the juggernaut.

I'm intentionally not quoting Scott because he changes his mind frequently and is therefor not reliable.

The "found egg" would be perfectly in line with the rest of the newer canon material, even without the Space Jokey.

On the mural; that sure looks like a xenomorph to me.

http://thewertzone.blogspot.com/2012/06/filling-blanks-tying-prometheus-to.html

It would have made more sense to imply/say that David created a Queen from Shaw and that's where the eggs came from? Or, if you follow Prometheus... she developed the first "Royal Facehugger" since she had also produced the "Trilobyte" instead of a Deacon/xenomorph....

It looks more Deacon like to me but I could be wrong.

It head appears to be more thinner than the classic Xenomorph.

@Tiwaz David said "No one will understand the perfection of my dreams. I found perfection here...I created it."

David may have meant by creating the perfect organism he found perfection. 

It would make no sense from a story telling point of view to insinuate that David created the xenomorph.

It would make the Alien franchise universe exponentially smaller and more uninteresting.

edit: also the derelict on LV426 had a spacejockey that was old enough to be fossilized (millions of years) and had a chestburster type injury.

While they are at it, we could always start demanding they insert a Herk Mondo type character to make things interesting, since RS is swaying in the way of audience complaints. Everything up till the final prequel films and maybe a reboot is sadly only going to be the way RS and Fox want it. In my opinion, it will be lightly touching on plot and focusing on Xeno action, the way current trends are going with audiences sadly. Personally I would rather see a more developed plot that ties up loose ends and not generate more questions, unlike what we have been getting.