We've never seen the monster before 1988, and there have been no references to it before that at all (Ancient Egypt aside, I'll get back to that later). We learn in LaFleur that the Dharma fence is to keep the Others out, not just the monster. Possibly, the monster was born after the events of 1977. Perhaps even because of them.
We don't really have any idea how the monster came to be. Maybe the monster was formed after the prevention/causing of the incident. Maybe it's the aftermath of exploding a nuclear bomb next to a pocket of radiation. Maybe it's Juliet, molecularily changed into a cloud of smoke (nuclear explosion + time-travelling radiation pocket + human vessel = monster?). Possibly it ties in with Jacob and his nemesis. Either way, i think it'd be a really cool beginning scene: picking up right where we left off (a surprising first for a season premiere), Jack+Co. are like 'wtf' after a flash of light and then the monster comes screaming out of the drilling shaft.
I just have a feeling that it's ALL going to come together in season 6, all the mythology: the incident (assuming whatever happened happened) creates the monster, time travel is involved which is why there's ancient egypt + the monster all tied up in the island mythology, possibly also backstory of Jacob + nemesis. In short, season 6 mythology ties together Jacob+Jacob's death+nemesis+egypt+monster+incident.
..now just get Widmore, Libby, Mile's powers, Hurley's powers, Hurley Bird, black rock, richard, claire's seagull, christian shepard, the white light locke saw, eko's last words "you're next", the pregnancy issues, the hostiles, healing properties, henry gale, eloise hawking, desmonds powers, the frozen donkey wheel, time discrepencies on and off the island, and the Jack-Kate-Sawyer-Juliet love square and it'd a great season warp-up.