In the beginning the beast is just something the kid with the mark thought he saw.
Lord of the Flies THE BEAST
In the beginning the boys think the beast is a snake.
The boys are afraid of the beast, but only Simon reaches the realization that they fear the beast because it exists within each of them. As the boys grow more savage, their belief in the beast grows stronger.
In the novel, the pig's head on a stick, covered in flies, is a horrific symbol of how far the violence has come.
The boys behavior is what brings the beast into existence, so the more savagely the boys act, the more real the beast seems to become.
The pig was killed by Jack and his hunters and the head is put on a stick as an offering to the 'beast'.
Through the chapters the image of the beast has evolved from an unspoken of, shapeless fear to a snake-like thing, to a gigantic water creature, to a mysterious ghost, and finally to a dark, moving form, secretly a dead man and his parachute.
The boys, fueled by fear, are the real beasts. Without rules and order the have killed 3 boys and taken on the persona's of savage's. Only when they are rescued they realize this.
The lord of the flies becomes both a physical manifestation of the beast, a symbol of the power of evil, and kind of Satan figure who evokes the beast within each human being.