The scene doesn't have to be literal as to screengrab it. It was obviously intended to look non-human on screen. It was also obviously intended to be ambiguous and perhaps unanswered. Just because it looks non-human doesn't make it not human – it could as well be metaphoric of what they imagined behind them.
As to what was under the blanked and then blurred, I think this is the stuntman who played the figure: http://www.imdb.com/name/nm0787449/
As to what was under the blanked and then blurred, I think this is the stuntman who played the figure: http://www.imdb.com/name/nm0787449/