It's a dead facehugger egg and a dead engineer (or maybe just his suit).
Eggs are an already centuries old weapon, thus is what the Derelict Engineer was fosilised.
Prometheus is not about the origin of the xeno, they already exist.
They are cheating with the Engineer's size,it will not be as huge as the one from ALIEN.
But it will work, thanks to some clever camera work.
It's an Engineer that risesin front of the sandstorm in the leaked trailer.