Your question is a little confusing, but I think you're asking how Percival's fear of the beast foreshadows what happens in the novel. If so, it's a good question. By chapter five of Lord of the Flies, the boys have been on the island for a while. Things haven't yet completely deteriorated, but the lines between the hunters (soon to be savages) and the rest are clearly being drawn. These boys are accustomed to being away from home, as they've been in boarding school; however, they're young and their fears are beginning to get the best of them. In one of the last assemblies on the island, Ralph scolds them all for being too lax and not obeying the rules they established. When the idea of a beast is broached by Percival, it is initially dismissed as ridiculous. The littluns, in particular, have been experiencing nightmares, and the most common elements on the island have taken on frightening aspects in the dark. One of the older boys brings up the idea that the beast can't be seen on the island during the day because it's actually at home in the water, rising up every night to frighten them. This image temporarily silences the group, then there is restless movement until Jack boasts that he will kill the beast if there is one. The boys are not reassured, and the chapter ends with the sound of a littlun crying in the dark.
This incident foreshadows the reality that there is a beast and it is them. It can't be seen, except in nightmares, because they don't recognize it in themselves. This admission that there might be a beast is brought to reality first by the Lord of the Flies and then by their own actions. Ironically, Jack is the one who claims he will kill the beast and yet is most representative of the worst in human nature--the beast in all of us.