Depending where you are in the book, you will find that a majority of the boys believe in the beast. At times, they think the pigs that travel the island might be the beast. Other times, the dead man hanging by his parachute is believed to be the beast. However, Simon sees for readers that this is not the case. At another time, a large mass is believed to be the beast. Once again, older ones discover this is not the beast. The little ones continue to believe passionately in the beast and they convince the majority of the older ones in the presence of such a beast. The tribal boys kill a pig and mount his head. This could be another allusion to the beast.
By the end of the book, readers realize that the beast is the evil within man. Much biblical allusion is given to the Devil, therefore it is ultimately up to the reader to decide if they believe in the reality of evil.