the smoke monster. . . .
when Richard visits Locke as a young boy, there is a drawing on the wall which clearly depicts the smoke monster.
i think there could be two possible reasons for this.
firstly, a time loop is happening so subconciously, John remembers the smoke monster even though up to this point in his current life he has not encountered it yet.
or, the smoke monster actually has some iconic meaning (death, fate etc) within the story and if John had had some sort of dream about, for example death, when he was young, this could explain the drawing.
or maybe there (probably) is some other explaination?
i do think, however, that this drawing needs explaining though in S6.