Who is the first beast?

If you are referring to 'Lord of the Flies' there never was a real beast. The littlun with the mulberry coloured birthmark on his face reported seeing a snake-thing, which he also called a beastie. Later Jack said that some of his hunters thought there might be a beast in the jungle, some sort of animal. Later still and Samneric mistook the dead body of an airman and his parachute for a beast with wings and claws. Unfortunately most of the boys mistook Simon for the beast when he staggered out of the jungle during a thunderstorm and they beat him to death. The true identity of the beast, as recognised by both Ralph and Simon, was simply the darkness, the capacity to do evil, which exists inside us all.