Well apparently in the EU The faceless AT-AT was pilotless machine designed by, you know the stormtrooper who found the droids bolt on Tattooine...well not him, the guy to his immediate right in that shot.... well anyway this "Shadow-Walker" was designed to match the motions exactly of its counterpart Walker as seen in the picture and wreck double the carnage....except it didn't have any weapons.

Their mission failed due to the fact, as Sluggo noticed, that due to it's counterpart that it was shadowing having only three legs, they both fell over.