I think that you are asking about the "beast" that some of the littluns fear. Some of them fear that it is a snake, not a "monster" like you might imagine. They think it is a snake because they see "creepers," or vines, in the jungle at night and their imaginations take over and they are scared.
Their fear introduces the idea of evil to the island. If you look at snake symbolism, it goes back to the garden of Eden story in the biblical book of Genesis. In that story, evil enters the garden when a serpent, which is twined in the branches of the tree of knowledge of good and evil, tempts Eve into eating the fruit. She then gets Adam to eat from this forbidden tree as well and the two of them are subsequently exiled from the garden because they disobeyed God. Golding's use of the snake in Lord of the Flies introduces evil (through fear) into the island society. Once that happens, the society soon breaks down, the boys become more and more uncivilized, and murders occur.