Where does Percival say the beast comes out at night?
In Chapter 5, Ralph organizes a meeting to get a few things straight. First, he chastises the boys (particularly Jack and the hunters) for allowing the fire to go out. The fire went out in the previous chapter and they missed a chance to attract the attention of a passing ship. Ralph also warns them about the danger of setting the entire island on fire. Ralph is frustrated. He realizes that something is "breaking" the boys up. He concludes that it is fear. So, he attempts to address their fears.
Ralph tries to get to the bottom of the rumor of the beast. He asks the littluns. Phil says he dreamed about the "twisty things" and woke up to see someone or something moving in the forest. It turned out to be Simon. Percival is brought forward but is reluctant to speak. Jack grabs him, Percival mumbles and then falls asleep. Jack informs them that Percival says the beast comes from the sea. Because the sea is dark and mysterious, this notion of the beast coming from the sea reinstates that fear:
The assembly looked with him, considered the vast stretches of water, the high sea beyond, unknown indigo of infinite possibility, heard silently the sough and whisper from the reef.
They fear the "beast" because it is mysterious. They fear the unknown: the unknown elements of the island and they may also fear what they are capable of themselves.