Shaft bows are only used in Russia, Finland, the eastern Baltic countries and, perhaps oddly, Sicily. It is used in single horse or troika (three horses in a row) harness - in the latter case it's put over the middle horse.
It acts as a spring and allows for a smoother start with faster horses. They're often painted. If you want a culture to do unusual harness - this is one possibility.
Shaft bows are generally wooden - metal ones have been tried, but tend to crack in the cold climates this harness is popular in.
Painted shaft bow on the middle horse of a troika. Image source: Wikimedia commons.