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http://en.wikipedia.org/wiki/Harness_saddle
A harness saddle is an element of horse harness which supports the weight of shafts or poles attaching a vehicle to a horse. Like other types of saddle, it lies on the horse's back directly behind the withers, often has an internal supportive framework, often called a saddle tree, and usually is secured on either side by a girth passing beneath the horse. Unlike riding saddles, it is an integral part of the harness and is not used as stand-alone equipment.
The harness saddle has at least two lines of evolution, both departures from the very ancient throat-and-girth harness . One line was developed as a refinement of that harness and involved a wide variety of martingale and breastplate type straps in front of the shoulder and between the forelegs, all attached to a saddle and girth. This line is documented in Byzantium in the 10th century and Khmer in the 12th century, and occurs today in Japanese plough harnesses and Indian tongas. The other line, far more successful, evolved in China from ancient withers-straps that originally were used with a breastplate-and-breeching harness without a girth. This was the Han departure from the throat-and-girth harness. A minority of 2nd century Han art shows this new breastplate-and-breeching harness with a girth added to it, and in that context ancient Chinese texts sometimes refer to the withers-strap as a saddle. This 2nd century harness saddle supported one of two arches attached to shafts, and had terrets through which the reins were carried.[7] The other arch resembles the shaft bow that is in use today in Finland and Russia.
A horse wearing a shaft bow also commonly wears a collar and a saddle. In a troika, only the center horse wears a saddle. The side horses may wear collars, breastplate-and-breeching, or surcingles.[citation needed] An example from 1912 shows a team of three horses in Russia, all wearing collars, the center horse wearing a shaft bow and harness saddle.