During an assembly in chapter 2, a littlun with a mulberry-colored birthmark timidly asks to hold the conch to discuss the terrifying "snake-thing" that he saw the previous night. With Piggy's help, the littlun with the mulberry-colored birthmark says that he witnessed a "beastie" in the dark forest. While the older boys initially dismiss the littlun's beastie as a figment of his imagination, the littlun insists that he saw it coming and going during the night and is worried about seeing it again. When Ralph argues that the beastie does not exist, Jack announces that even if it does exist he and his hunters will kill it. The beastie is significant to the littlun with the mulberry-colored birthmark because its presence fills him with terror. While the identity of the beast remains ambiguous at this point in the story, it is real and threatening in the littlun's eyes. He does not want to sleep by himself and looks to Ralph and the older boys for protection. The topic and identity of the ominous beast is brought up once again during an assembly in chapter 5 and completely alters the boys' behaviors once Samneric mistake a dead paratrooper's corpse for the beast. Out of all the boys on the island, only Simon is aware of the beast's true identity and recognizes it as mankind's inherent wickedness.

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.