The imaginary beast frightens all the boys. It stands for the primal instinct and savagery at everyone possesses. As the boys grow more savage, their belief in the beast grows stronger.
The Beast a Motif in "The Lord of the Flies"
At first, the beast seems to be triggered by the savage hunters lead by Jack. After their first kill the boys begin telling stories about what else is in the jungle.
Later, a paratrooper (dead) lands on the island. The boys can hear the noise made by the chute and fear engulfs them; they think it is the beast.
One of the older boys (Simon) has a dream about the wild boar, who's head was staked by the 'hunters' as a sacrifice to the beast.
Simon finds the parachute and realizes that the dream is not about a tangible beast but one that is living within the boys.
Motif: The boys, fueled by fear and anarchy, are the real beasts. Without rules and order they have killed 3 boys and taken on the persona's of savage's. Only when they are rescued do they realize this.