Beastie symbolizes that not everything is what it seems.
Beastie started off as a bad dream of a snake like thing to little creepy crawlers and loud noises to a god like being and then finally a part of everyone.
Pg,35 "He wants to know what you're going to do about the
Snake-thing. " Ralph laughed and the other boys laughed
With him. The small boy twisted further into himself. "Tell
us about the snake-thing." "Beast is? " "A snake-thing. Ever so big. He saw it."