In chapter two, the littlun with the mulberry-colored birthmark mentions that he saw a "snake-thing," which he later refers to as a "beastie." When Ralph dismisses the beastie as a figment of the littlun's imagination, the boy says that the beastie came at night and wanted to eat him. Ralph continues to insist that the littlun was simply having a nightmare, but the boy continues to elaborate on the identity of the beastie. Jack then seizes the conch and confidently says that if there is a snake-thing or beastie on the island his hunters will kill it. He also says that while they are hunting pigs, his hunters will look for the beastie too. Jack's response is typical and illustrates his affinity for hunting and killing. As one of the most savage boys on the island, Jack views hunting and killing the beastie as an admirable challenge that he is willing to accept.
Jack has just discovered "the hunt" and is about obsessed with it. So if there is a beast (which he doesn't really believe at this time), he will hunt it. Piggy knows scientifically that it's not possible. Simon knows that there is no beast, and Ralph even says that it's not possible. But Jack is the aggressive one and wants to HUNT it.
"Ralph's right of course. There isn't a snake-thing. But if there was a snake we'd hunt it and kill it. We're going to hunt pigs to get meat for everybody. And we'll look for the snake too--"