In Chapter 2 of Lord of the Flies, explain how the beastie was important to the boy with the birthmark.
When the boys gather together after hearing Ralph's conch, the younger boys push the boy with the birthmark to step forward. He tells the boys that there is a beastie in the woods, which he describes as a "snake-thing" and says that it wants to eat him.
Ralph dismisses it as a nightmare and the older boys laugh. The younger boys seem to be somewhat afraid though.
This whole episode is what starts the fear on the island. The beastie comes up again in Chapter 5. Of course we find out that the beast is not real, only their fear is real and later the savage nature of the boys becomes the beastie. The line "The boy twisted further into himself" implies, as we find out later, is the beast within everyone.
The mulberry-faced boy is lost in the fire later in this chapter.