The dead man in Chapter 7 of the Lord of the Flies was the parachutist who ejected from a plane when it exploded.
The dead parachutist lands on top of the mountain. He looked to be alive because of the strong winds that swirled over the corpse and its parachute; making his body bow forward and sink over and over.
The boys saw the corpse and its movements and they thought they were confronted by a live, predatory monster.