The "thing that bows" is in chapter 9 - it is the lifeless figure of the parachutist who is entangled in the tree branches on top of the mountain, where the signal fire used to be kept. This is the same figure that Sam and Eric saw when they were tending the fire, causing them to run down the mountain to the older boys, declaring that they had seen "the beast."
Simon discovers this figure when he climbs the mountain to discover the truth about "the beast" - as he approaches the top of the mountain, he "could see something happen, a flicker of blue stuff against brown clouds," and as he gets closer, he sees "a humped thing suddenly sit up on top and look down at him" (Golding 135).
Simon does not give in to fear; instead, he hides his face and continues up the mountain toward the figure. Golding describes what Simon sees:
The flies had found the figure too. The life-like movement would scare them off for a moment so that they made a dark cloud round the head. Then as the blue material of the parachute collapsed the corpulent figure would bow forward, sighing, and the flies settle once more (135).
Upon discovering the truth, Simon sets the parachutist free by disentangling his lines and allowing him to drift out to sea. He then descends the mountain with the goal of telling the others the truth; however, he is never able to deliver his message because he is attacked on the beach.