Where is this statue on horseback from?
I believe it's the Imperial General Staff Headquarters. (Photo from Geospatial Information Authority of Japan for comparison below. Similar images in the postcards scanned in the link.)



As for the statue it seems to be of the founder of the General Chief of Staff, Prince Arisugawa Taruhito. While the current statue is now currently located in Arisugawa Memorial Park in Minami-Azabu, Minato, Tokyo, the General Staff Headquarters was located across the palace moat as seen in one of the links.