What does Ralph say he is afraid of?

Ralph might have been afraid of a "monster" lurking on the mountain but I don't think he took this threat entirely seriously. In a broader context I think Ralph's real fear is Jack increasingly undermining his authority. Jack is openly questioning Ralph. Ralph feels the other boys gravitating to Jack and the chaos he embodies.

Ralph is afriad of not being able to get off the island or being rescued. His fears throughout the novel increase. Ralph is afraid that the signal fire will not stay lit. He is also afriad that chaos within the boys gets worse... Ralph notices the savagery within the boys and their group starts get worse and eventually corrupt.