Did all the boys belive they had seen a beast near the pink granite
I believe the incident you're referring to in Lord of the Flies is the time Samneric were supposed to be tending the fire on the mountain but fell asleep, only to wake up to a strange sight. They saw something moving, and they heard a kind of whooshing sound. They were too frightened to investigate; they simply ran down the mountain to the beach to let the others know what they had seen. By now, the beast had fangs and had chased them the entire way--pure figments of their imagination, but believable because of their scrapes and cuts (which they got as they tore through the brush on their way to the beach). The twins believed they had seen the beast, and the young boys believed their story. In fact, of course, this was the dead parachutist whom Simon will later release from his entangled chute. With the vivid imaginations of young children, especially after the series of events which has happened to them on this island, they did believe they had seen the beast. We know better, though, and Simon will also soon learn the truth.