Here are the opening lines of Chapter 8, and the boys' terror is absolutely self-evident:
Piggy looked up miserably from the dawn-pale beach to the dark mountain.
“Are you sure? Really sure, I mean?”
I told you a dozen times now,” said Ralph, “we saw it.”
“D’you think we’re safe down here?”
“How the hell should I know?”
Ralph jerked away from him and walked a few paces along the beach.
Jack was kneeling and drawing a circular pattern in the sand with his
forefinger. Piggy’s voice came to them, hushed.
“Are you sure? Really?”
“Go up and see,” said Jack contemptuously, “and good riddance.”
“The beast had teeth,” said Ralph, “and big black eyes.”
The boys are terrified that the beast is at the top of the mountain. This is because first Samneric, and then the rest of the boys, saw the parachutist's body lying at the top of the mountain, and his parachute flapping in the wind, which arrives in Chapter 7, "Beast From Air".
It looks like a beast, it casts an ominous shadow - and the boys' imaginations and fears fill in the rest. It is, of course, entirely harmless - just the corpse of a parachutist.