What question does the littlun with the birthmark raise? How do Ralph and Jack answer it? What is the significance of "Fire on the Mountain"?

What you are talking about happens in Chapter 2.  The
littleun with the mulberry colored birthmark raises the question of the "beastie."  He
says it's a snake-like thing and he is afraid of it.

Ralph
tells him that there is no such thing.  But Jack starts talking about hunting
it.

To me, the chapter is given this title because of what
happens at the end.  The boys set the fire, but they are so careless that it gets to be
a huge fire and it ends up killing the boy with the birthmark.