In Sci-Fi and horror I've always questioned what this creature was. My dad called me about this recently so I did some research.
The Space Jockey is discovered in Alien when the commercial starship Nostromo sets down on the unsurveyed moon LV-426 to investigate an unknown signal. The crew finds a crashed derelict spacecraft with its fossilized pilot—the Space Jockey—inside.
Scott suggests in his Alien DVD commentary that the Jockey's ship was a "bomber": alien eggs could be dropped on an enemy planet, and the aliens would proceed to kill the population as they spawned. According to Cameron, the Space Jockey's craft picked up alien eggs and the pilot became infected by the dangerous cargo; the ship landed on LV-426 and the Space Jockey transmitted the signal as a warning. In Foster's Alien novelization, Ash describes the Space Jockey's race as a noble people and hopes that mankind will encounter them under more pleasant circumstances. It also states that they were larger, stronger and possibly more intelligent than humans. Foster's novelization states that the Jockey was trying to warn humans away from the aliens.