What is the symbolism behind the Beastie in Lord of the Flies?

The Beastie represents the boys’ fears of the unknown. 

When the little boys first discuss the Beastie, no one wants to take it seriously.  It is just a joke, something that the small boys are afraid of.  It comes up at an assembly meeting. 

“He wants to know what you’re going to do about the snake-thing.”

Ralph laughed, and the other boys laughed with him. The small boy twisted further into himself.

“Tell us about the snake-thing.”

“Now he says it was a beastie.” (Ch. 2)

Ralph tries to explain to the boys that a snake thing could not exist on an island of that size.  The older boys try to dismiss the littleuns’ fears as a bad dream or the fears of young children away from home.  They do not want to admit any fears themselves.

Jack uses this as yet another opportunity to undermine Ralph.  As Ralph tries to assure the little boys that there is no Beastie, Jack takes the conch and tells them that if there is one he would take care of it for them.

Jack seized the conch.

“Ralph’s right of course. There isn’t a snake-thing. But if there was a snake we’d hunt it and kill it. We’re going to hunt pigs to get meat for everybody. And we’ll look for the snake too–” (Ch. 2) 

Ralph is annoyed because Jack contradicted him and also implied that he could take care of something that Ralph could not.  It is another example of the difference between the two of them and the constant struggle for leadership.  Their styles are different, and Jack is always trying to be theatrical while Ralph is always trying to be practical. 

The Beastie comes to represent the fear of the unknown, not just for the littleuns, but for all of the boys.  Jack meets the unknown head-on, while Ralph makes plans.  The Beastie never goes away.  It is always in the back of everyone's minds.

“They talk and scream. The littluns. Even some of the others. As if—”

“As if it wasn’t a good island.”

Astonished at the interruption, they looked up at Simon’s serious face.

“As if,” said Simon, “the beastie, the beastie or the snake-thing, was real. Remember?”

The two older boys flinched when they heard the shameful syllable. Snakes were not mentioned now, were not mentionable. (Ch. 3)

When the children attack the Beastie, it turns out not to be the beast after all.  It is Simon, coming to warn them about the parachutist he saw that frightened him.  Beasts are everywhere, but they are all manifestations of the boys' fear.