When the boy with the mulberry-colored birthmark steps forward under pressure from some of the other little boys, he tells a story about seeing a "snake-thing" that he saw the previous night in the dark. At one point he also refers to it as a beastie.
This is the first reference to the "beast," what will come to be a central theme in the novel. At this point the beast is external, something the boys fear but that they cannot even accurately describe or picture. The little boy says it was a "snake-thing" but Piggy quickly points out that it would be impossible to have such a large predator on an island so small. But the boy insists that he saw it and that it wanted to eat him.
It quickly becomes apparent that this beast is a stand-in for the boys' fear. They are stranded on an island without adults, without rules, without a concrete hope for rescue and so they are afraid. It also begins to symbolize their fear of what they are capable of. The beast represents the force that will push the boys to violence and savagery once the trappings of civilization have been washed away.