The "beast from air" in chapter 6 is ironic because in chapter 5 the boys believe there is a "beast from water," but those fears are groundless. The boys at the beach fall sound asleep after having been startled by a littlun having a nightmare. Meanwhile, Samneric, the twins, are keeping night watch on the mountain, ostensibly keeping the fire burning. However, they fall asleep as well, and while they sleep, an air battle rages overhead, part of the world-wide nuclear war that resulted in the boys' landing on the island. A paratrooper, killed before or during his jump, lands on the island. The parachute fills with wind, causing the corpse to be pulled up the island.
Samneric wake up and tend to the fire. While Sam's back is turned to the paratrooper, Eric sees the movement and hears the sound of the fabric. Eric screams at Sam, and he comes around the other side of the fire to see it. Hugging each other tightly, they are frozen in fear for about ten seconds. Then they careen down the mountain and wake Ralph up to tell him they have seen the beast.
The "beast from air" that floats down to the island is a dead parachutist from an air "battle fought fought at ten miles' height" above the island. He is discovered by the twins Sam and Eric who, "as though they ha[ve] but one terrified mind between them," rush back to tell Ralph and the other boys.
The figure of the dead parachutist is important because it is a tangible representation of what the boys fear the most. Since his death is caused by violence in the adult world, the parachutist forshadows additional bloodshed from the real beast--the "beast" within the boys themselves. Much like the "grownups" far away, the boys will begin a civil conflict that will end in death.
the beast is tim and sams dad.