The idea of a beast being present on the island is first raised in Chapter Five ('Beast from Water'). Ralph has called a meeting, essentially to assert his leadership after the disastrous neglect of the fire by Jack and other boys. However, he finds the concerns raised in the meeting rapidly get out of hand.
At first the idea of a beast is vaguely connected to the little boys' dreams, and the older boys are able to dismiss this. Jack insists, "there is no beast in the forest" (p.104), whilst Piggy attacks the question scientifically, "Life....is scientific, that's what it is.......I know there isn't no beast -not with claws and all that..." (p.105). Soon however the talk of the younger boys swings back to the possible existence of the beast, with one stating that "the beast comes out of the sea" (p.109), and another backing this up by stating "Daddy said they haven't found all the animals in the sea yet" (p.109). This causes the whole body of boys to become spooked and it isn't helped when Simon as an older boy suggests the beast could be real, but is unable to explain his comment. In fact both Simon and Piggy are coming close to the real truth about 'the beast' but can't quite grasp it. Piggy comes tantalisingly close to understanding that the beast is actually the savage within the boys when he talks about there being no fear "Unless we get frightened of people" (p.105).
The meeting becomes a disaster for Ralph's leadership. Not only is everyone left uncertain about the existence of a beast, but Jack and his hunters are clearly ready to break away from the rest of the group. Jack openly defies Ralph at the breakdown of the meeting and rejects the rules, but more significantly his appeal to the boys as a protector against a possible beast is greatly advanced by his cry of "If there's a beast, we'll hunt it down!" (p.114).