In a physical sense, the beast is Simon. Simon had gone to the special place where he'd had a fit and ended up having his vision of the Lord of the Flies. After the vision, he proceeded up the mountain where he saw that the beast on top of the mountain was in fact a dead pilot.
He decides that he ought to tell the rest of the boys what happened so he makes his way down to the beach and crawls out of the jungle to tell them. But the boys mistake him for the beast and attack and kill him in a frenzy.
In this way, he also represents their fear, the real beast that is inside of each of them as the Lord of the Flies told Simon. Their descent into the madness that allows them to kill one of their fellows accompanies this emergence of the "beast" from the forest.