Except he definitely didn't create the creature from scratch. A form of creature existed long before David got his hands on the accelerant as displayed on the mural in Prometheus. Also, doesn't David say something to the effect of the engineers had their wolf and tried to banish him and now he had created his wolf in the advent short? There is Also, if you look carefully, in David's layer, a painting of the original Alien ovomorph hanging to the left of a painting of his new version of the ovomorph.
I imagine that yes some ancient form of the alien creature born from the Engineer's DNA accelerant mixture was running amok on LV-223. In my opnion anyway.