Called horo ("cloak") this very unusual protection was used during the Sengoku Period by elite special troops of Oda Nobunaga. The most famous of them, represented in this statue from the Oyama Shrine in Kanagawa, was Maeda Toshiie, the leader of the Aka Horo Shu, "Red Cloak Horsemen".
The armor was in fact a heavy cape placed on a bamboo frame (similar to a crinoline), covering the rider's back. It was a very innovative armor, light enough not to bother the rider, designed to protect him from the arrows: even if an arrow was piercing the cape, it was slowed down enough to be harmlessly caught in the empty space.
EXIF info:
Nikon D90
Lens: 8-16mm F/4.5-5.6G
Focal Length: 14mm
Aperture: F/5.6
Shutter Speed: 1/320s
ISO Sensitivity: ISO 200