Was it David or the Engineers who created the Neomorph?

[SiL]

During press for Prometheus Scott said the derelict and its cargo were already on LV-426 waiting to be found.

Then he changed his mind.

Saying there was any real plan or direction is being very generous.

[David Weyland]

Sure, it was just in context of the topic question and wider Covenant meanderings that it was what I conceived of the lore & its origins (Watching that film)

[Corporal Hicks]

The prequels direction I reckon was to have the xenos or at the very least the LV426 xenos to be by David's hand
But unfortunately that's sacrilege to the majority so it'll be made ambiguous again to keep everyone happy

It certainly was the intent when working on Covenant. But there's still plenty of room left within the actual film for that to not be the case.