that's a very good question! is it a prototype or just the result of a lego designer getting creative? what is really weird is that some parts, like the wedge of the nose of this thing, were never produced in this colour, according to bricklink cat. I don't think TLC would set up a production line just for this model, so...
i like the legs of this thing, they really look powerful, especially compared to the legs of the AT ST...
could there be a UCS set involved?
Maybe it something from the EU. I mean there is nothing like that in the movie.
Did you notice the minifig was a AT AT pilot and the one at the back a normal storm trooper