Yeah, I'd like to think it's an entirely different creature all-together. And like you said, the Derelict spaceship in LV 426 was crashed thousands of years prior to the ship in Prometheus. Plus, the Derelict ship had Xeno eggs in stasis... so, with that said, there's no way that the Deacon could be the progenitor of the race when the Xenos came before it. It just wouldn't make sense...
I still think that the mural in the Engineer chamber was a Xenomorph. The Deacon seems like just a freak-of-nature accident that was caused by David.
I still think that the mural in the Engineer chamber was a Xenomorph. The Deacon seems like just a freak-of-nature accident that was caused by David.