It is interesting that the Beast when it first appears is treated with scepticism. Ralph flat out denies the existence of the beast, typically in character for the representation of civilisation in the novel. Piggy tries to find a scientific explanation for it, and Jack initially sides with Ralph until he realises he can play on the fear of the boys of the Beast to seize power by the protection he can offer them by hunting the beast. It is only Simon that is able to put his finger on the truth - that the beast represents the evil that is in them all, that it is something that comes from within them all: "What I mean is... Maybe it's only us..."
What is clear though is that the Beast therefore stands for the savage inner core that exists within all humanity. The boys are terrified of the beast, but it is only mystical Simon who eventually arrives at the truth. The reason for their terror is that the Beast exists within each of them. As the boys continue to slide into barbarism, committing ever greater acts of savagery, it is key that their belief in the beast strengthens. This continues until the end of the novel, when the boys are treating it like a god, leaving it sacrifices. It is the actions of the boys that creates the beast, so the more savage the actions, the more fervent the belief in the beast becomes.