One really important thing to remember about the discovery of the dead parachutist is that the boys do not know that a parachutist fell on the island. This is an example of dramatic irony, a literary device that authors use to make the audience aware of something while keeping the characters unaware. Only the readers see the parachutist fall at the beginning of Chapter 6; none of the boys do.
As a result, when the boys climb the mountain to see what SamnEric have reported as the beast, their expectation is that of a beast. When Ralph sees the dead pilot, he finds "himself taking giant strides among the ashes, heard other creatures crying out and leaping and dared the impossible on the dark slope; presently, the mountaintop was deserted, save for the three abandoned sticks and the thing that bowed" (Golding 95). Using context clues, the reader can see that after the pilot's parachute forces his body to sit upright, the boys cry out and run away.
One detail I feel it is important to add is that in this scene, Piggy is not present. Instead, Roger accompanies Jack and Ralph to the top of the mountain.