The general wisdom is that horses travel better if they don't see the scenery or other traffic going past, which is why horseboxes and trailers tend to be mostly solid wall, with vents higher up.
That said, some equines don't really mind having a look out. I've followed trailers where the horse or pony is enjoying a haynet whilst watching out of a window, and at least one converted Renault Trafic van which had a pony facing the rear, watching me through the rear windows!