The "beast from air" is the body of a dead pilot. The body, attached to a parachute, has floated down to the island from a fighter plane overhead. It lands on top of the mountain, and Samneric -- who were tending the fire -- see the body and think it is a beast. Samneric wake up Ralph. The boys gather around. Jack wants to hunt the beast.
One of the biggest dangers that people face when alone in the wild is panic. This is true for the boys, as well, when they begin to believe they see beasts. A beast symbolizes the evil that is in all of us. It also symbolizes that irrational fear of the unknown.