You had to look fast and intently to see the 'monster." There's a theory on the Lost message boards that "it" is really swarms of nanobots a theory which totally makes sense if you saw the black cloud that flew away after they threw the dynamite into the tunnel behind Locke.

Lots ot theories on the Lost boards (go to ABC.com and follow the links) .... I can't wait till next season! I will be getting the DVD when it comes out.