The boys' idea of the beast is as undefined as their own fears. As the novel progresses, their idea of the beast evolves as their own fears concerning the island deepen and change. At first, the boys, particularly the littluns, see the beast as some sort of snake-like thing, and their simple fear is written off by some of the older boys as a fear of the jungle and the creepers. Later, when the boys discuss the beast, the older ones begin to buy into the fear of the unknown--that there might actually be some sort of horrific monster lurking about the island unseen.
In the chapter, "Beast from the Water," the boys suggest that perhaps the beast hides in the water, because they are unsure of where it could hide on the island; there are potentially unidentified sea creatures, according to Piggy. Then in the following chapter, "Beast from the Air," their worst fears about the beast take shape and are given a horrific definition in the form of the parachutist who floats onto the island and gets hung up in the trees, taking the shape of a hulking, bulging creature on the side of the mountain.