what do you think is the significance of the "snake-thing" or "beastie"?

On a literal level, the "beastie" represents a monster that lurks in the jungle at night. On a metaphorical and symbolic level, the beastie represents fear and the evil that exists in the boys themselves. Simon points this out when he thinks:

However Simon thought of the beast, there rose before his inward sight the picture of a human, at once heroic and sick. ch 6