Find descriptions of "the beastie" as you read.

"A snake-thing. Ever so big. He saw it."

"You did, the other day. You said they dream and cry out. Now they talk--not only the littluns, but my hunters sometimes--talk of a thing, a dark thing, a beast, some sort of animal.

From the text, we can infer that th "beastie" is a culmination of the boys' fears.