The Lord of the Flies is a pig's head on a stick, which is severed from the body of a sow which is hunted and killed by Jack and his hunters. Jack puts it on a sharpened stick, and leaves it as a gift to the beast:
Jack spoke loudly.
“This head is for the beast. It’s a gift.”
The silence accepted the gift and awed them. The head remained there, dim-eyed, grinning faintly, blood blackening between the teeth. All at once they were running away, as fast as they could, through the forest toward the open beach.
The pig's head then, obviously attracts flies, and hence becomes, in Simon's mind, "The Lord of the Flies". Simon has an odd prophetic, spiritual sense, and his mind seems to personify it as a representation of the beast:
“Fancy thinking the Beast was something you could hunt and kill!” said the head. For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. “You knew, didn’t you? I’m part of you?"
Moreover, "Lord of the Flies" is a translation of "Beelzebub", a word of the devil. So it really is the "beast", the evil within man.
The parachuted man is, like the Lord of the Flies, just an innocent happening. Nothing scary. But the boy's imaginations and fears, just as Simon's did to the pig's head, make it into a beast. It's all in the mind - and the "evil" of the beast is too. What makes them scared - and what makes them behave badly - is the darkness of their own hearts.
In a way, both the paratrooper and the pig's head represent the beast. The title "Lord of the Flies" actually means "Satan" or "the devil" Also note that the pig's head was put on the stick by Jack, who committed an act of evil, of being a beast, by killing the mother pig. The paratrooper is the ironic "sign" that the boys recieve from the adult world. It is showing that the beast exists, not only on the island, but out in the rest of the world as well, in the form of evil inside of people. The paratrooper crashes as a result of fighting overhead, because of the war that is going on.
The parachute guy was mistaken for the beast for a long time throughout the story. But then simon finally goes up to it and finds out that it is just a parachute guy that has died. He trys to bring back some of the parachute to show everyone that that wasnt the beast but it was dark out and pouring down rain. So they all mistake him for the beast and kill him.