The concept of the "beast" is what best points out the different types of boys on the island in "The Lord of the Flies." That some boys imagine the beast in various forms is typical of simpler people who must have a concrete form for concepts that exist. On the other hand, if there is no concrete form, then some simply dismiss the concept as non-existent. For instance, in Chapter 5 the savage Jack discredits this beast by saying,
Well then--I've been all over this island. By myself. If there were a beast I'd have seen it. be frightened because you're like that--but there is no beast in the forest.
The more rational type like Piggy expresses the idea as this:
Life ... is scientific, that's what it is....I know there isn't no beast--not with claws and all that, I mean--but I know there isn't no fear, either"
Rationally, Piggy recognizes the resulting fear from the imaginations of the littl'uns, but does not believe in the beast, per se.
Still others mix real and imaginary as Phil does when he describes his dream:
Last night I had a ...horrid dream, fighting with things. I was outside...fighting with things, those twisty things in the trees.
It is only the intuitive Simon who understands that the beast is neither concrete, rational, nor a dream: It is the evil in the soul of man.
The beast, of course, which doesn't exist, changes in the boys' imaginations. In Chapter 2, it begins as a "snake thing":
Ralph laughed... The small boy twisted further into himself.
“Tell us about the snake-thing.”
“Now he says it was a beastie.”
“A snake-thing. Ever so big. He saw it.”
“In the woods.”
Golding is always careful to let the reader know what the "beast" really is: in this case, it's simply vines:
“He says in the morning it turned into them things like ropes in the trees and hung in the branches."
In a later meeting, there's a suggestion that the beast comes from the sea, or even that the beast might be a ghost. Simon, in the same meeting, has it right: there is a beast, he says, but “What I mean is. . . maybe it’s only us.”
And the same thing happens later when the parachutist falls to the island, "beneath a parachute, a figure that hung with dangling limbs." This becomes, in the eyes of SamnEric,
“There were eyes—”
“We ran as fast as we could—”
“Bashed into things—”
“The beast followed us—”
“I saw it slinking behind the trees—”
The key point here is that there is no consistency to the way the beast is imagined. Which makes sense: because the beast itself is entirely imaginary in any realistic way. The darkness which hides the "real beast" is, as Golding points out in the final paragraph of the novel, "the darkness of man's heart".