Both Piggy and Simon deny the existence of a physical "beast" on the island. However, Simon believes that though not physical, there is a beast. Such a beast is the one that is inside all of the boys, and is inside all those of humans. The innate evil of mankind.
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He says in comes out of the ocean at night. :P
In Ch 5 of Lord of the Flies, Ralph says there are no monsters on the Island. Piggy agrees with him. Simon says the beast is within them.
ralph to piggy
Simon is an epileptic and during an epileptic fugue he has an internalised conversation with the evil which he knows dwells within us all. He mentally projects this inner beast onto the pig's head on a stick which Jack has left as an offering for the imagined beast.
It was homage to the beast and this, being the Lord of the Flies later, is related to the devil. Lord of the flies in Hebrew has a similar name to Beelzebub, which is the devil. It signifies that the devil is all part of us, when he says he is part of them to Simon during one of his seizures.