Samneric, being so young and so terribly naive, have got it into their head that the dead parachutist is some dangerous beast lurking on the island. In their irrational fear, Samneric even imagines that the said beast clawed at Eric's face as it chased them. It didn't of course, but then the mind plays tricks in the dark, especially when you're frightened. Jack's more than happy to believe Samneric's story; it's in own his interests to believe it, for if the boys are in a state of constant fear over a supposed beast on the island, they will be more likely to turn to him for protection.
But Simon, for one, is not convinced. If there really were a beast roaming around there'd be visible signs like tracks. Besides, if this beast is so fearsome then how come it couldn't catch up and hunt down two scared little boys? The idea that it was able to claw at Eric's face but not actually manage to catch him is utterly ridiculous.
Simon knows that the beast exists only in the boys' fevered imaginations; it just isn't real. The problem is that he lacks the ability to put his acute insight into words, and even if he could, it's unlikely that anyone would pay much attention to what he had to say. As countless examples from history teach us, once collective fear takes hold, it proves impossible to break its deadly grip with appeals to reason and logic. Besides, Simon's understanding of the beast as the evil that lurks within each and every one of us, is unlikely to go down to well with the other boys. No one likes to be told that they harbor evil deep within their souls, even if it does happen to be true. Perhaps it's just as well that Simon cannot articulate precisely what it is that his acute intuition and psychological insight tell him.