During Chapter 2, entitled, Fire on the Mountain, Ralph holds an assembly to assure the boys that the island is a good place. A nameless six-year-old boy with a mulberry-colored birthmark on his face steps forward and tells the group he saw a "snake-thing" or a "beastie" hanging in the trees during the night.
This point in the novel is really the source of all the boys' problems on the island. Not only is Golding introducing the idea of a beastie, but symbolically this is the introduction of fear, as the beastie represents fear of the unknown.
Later, the fire at the end of the chapter engulfs the foliage in flames that look much like the snake-thing the boy with the birthmark spoke about. This moment brings the fear to fruition--it now seems possible that a "beastie" does exist. The growing fear of a beast develops throughout the remainder of the novel until it both consumes and controls the boys.