The first "real" beast the boys see on the island comes up in ch. 6 when Sam and Eric are watching over the fire on the mountain. As they are sitting and talking, Eric sees something that catches his eye and quickly calls out for Sam to look:
The intensity of Eric's gaze made the direction in which he looked terrible.... (Sam) scrambled round the fire, squatted by Eric, and looked to see. They became motionless, gripped in each other's arms, four unwinking eyes aimed and two mouths open. Far beneath them, the trees of the forest sighed, then roared.... Fifteen yards away from them came the plopping noise of fabric blown open" (91).
The boys don't stay to figure out what they have seen - they immediately take off running and report to the other boys below:
"We've seen the beast with our own eyes.... It was furry. There was something moving behind its head - wings. The beast moved too -" (92).
What the boys have actually seen is the parachutist whose plane was shot out of the sky above at the beginning of the chapter in an act of war. The parachutist drifted to the island and became entangled in the trees. It is not until chapter 9, when Simon climbs the mountains and discovers the truth, that the parachutist is set free.