The beastie is a creature the boys claimed to have seen on the island. Jack and his hunter put a pig head on a stick as a sacrifice to it. However, during their feast Simon discovers the "beastie" is actually the dead body of a parachuter stuck in the trees. The pig head then tells Simon that the true monster is inside of the boys. This is the result of Goulding's (the author) metaphorical build up throughout the book. Examining it closely you can find a lot of symbolism Goulding wrote about the evil inside of man kind, how it builds or grows, and just what we are truly capable of doing to eachother.
