The Beastie

In the mind of the littluns it is an animal, a snake more specifically. Ralph believes they're just little kids with too much of an imagination, or a series of nightmares. Simon connects the beastie to everyone, their selves mostly since everyone has the power of cruelty and becoming evil. Piggy believes science, without facts/evidence it doesn’t exist it doesn’t make sense. Jack has a different mindset from all, no matter what it is he is going to kill it. There is no fear or sympathy in his soul for this beastie

The beastie, it can be fear of themselves. Fear of the savagery that might be coming towards them. A lot is slowing changing for them. The beastie is not an actual animal, nor human. It's more of a characteristic they're afraid of.