In Chapter 6 ("Beast from Air") what do the boys discover at the end of the island and how does it make them feel?
Samneric, on watch, see a half-version of the dead parachutist who has landed on the island at the very start of the chapter: they think they have seen the beast. Golding is quite clear that they have only seen the parachutist:
Fifteen yards away from them came the plopping noise of fabric blown open.
The "sighting" of the beast creates absolute terror. Samneric rush back to alert the others:
'Ralph! Wake up!' The leaves were roaring like the sea. 'Ralph, wake up!' 'What's the matter?' 'Wesaw-' '-the beast-' '-plain!'
'Who are you? The twins?'
'We saw the beast-'
Even in Ralph and wise, clear-headed Piggy, this sighting provokes untold terror, and leads to the violence which Jack instigates as a solution to the "beast's" presence. Golding, in fact, marks Samneric's news as the arrival of the "beast" into the boys' minds: Ralph and Piggy
...lay there listening, at first with doubt but then with terror to the description the twins breathed at them between bouts of extreme silence. Soon the darkness was full of claws, full of the awful unknown and menace.