What do the boys think the beast is?

From the text:

“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. I’ve heard. You thought not, didn’t you? Now listen. You don’t get big animals on small islands. Only pigs. YouonlygetlionsandtigersinbigcountrieslikeAfricaandIndia—”