Respuesta :
Simon shows a different perspective then the other boys, he represents innocence symbolically, he sees that the Beast is really them, they're always arguing, they can't govern themselves, they might even fight to the death just to determine a leader. The real beast is their own ignorance. Simon sees that.
I think that the beast they saw was one of them and the beast being "only us" refers to them as they have become. They were once human but in being trapped for so long alone in the wild they have become the beast. They are no longer just little boys but have reverted to becoming the wild beast within. They have given in to their own animal nature for the sake of their survival.