You missed the whole movie then.
The Elders, her father, Sigourney Weaver, and the others were dressing up as the monsters as a form of control. This invoked fear into the minds of the villagers and caused them never to think about ventuing out into the woods, in fear of their lives. They would periodically give some sign to frighten them, such as entering the village and marking the doors or skinning the animals (or so we believe).
Also, only the Elders and now, Ivy, knows that they are really in present day. Everyone else is suseptive to the form of control.
Noah (Brody) wasn't the monster at all before then. He found a hidden suit in the quiet room where he was being kept and escaped in an underground passage. After escaping, he went after Ivy because he was mad of her hitting him.
Hope I helped.