Ema are small wooden plaques on which Shinto worshippers write their prayers or wishes. The ema are then left hanging up at the shrine, where the kami (spirits or gods) receive them. These votive plaques date to the reign of the Emperor Suinin, the ancient leader that instituted the use of clay images or haniwa instead of live burials when tombs were built for the wealthy. Horses previously interred then came to be gifts to the shrines. These "shimme" were literally thought to be horses for the god to ride. It wasn't long before those "gift horses" became more trouble than they were worth. The next logical step then was the making of replica mounts on the same idea as those ordered by the emperor in years not long past. Shimme are still present in some of the largest shrines where they lead a life of luxury in stables situated within the grounds of the god's earthly abode. But invariably, a second building is not far away--the ema-do, a repository specifically constructed to hold and display the many votive plaques gifted the shrine over the years. The term EMA literally means picture horse but no longer is it exactly accurate. The earliest such pictoral representation were no doubt just horse drawings but the range has widened considerably since then without a change in name. See Amaury Saint-Gilles. "Mingei: Japan's Enduring Folk Arts". Tuttle Publishing, Tokyo (1990). pp 156-157.