When looking at statues and artwork of famous military leaders, the horse is often given one of three stances: all legs down if the rider survived, a raised leg if the rider was injured but victorious, and both front legs raised if the soldier fell in battle, or was defeated.
When did this artistic custom regarding the horse appear and become somewhat codified?
EDIT: I am basing my question largely from US 18th-19th century monuments, in particular the Revolutionary and Civil War.
It's actually a myth that statues follow this format. While there are numerous examples that do, there are more that don't. For example, two of George Washington's statues (from 1860 and 1959) have one hoof raised, but he died peacefully at home and didn't sustain any major injuries during battles. Stonewall Jackson's monument in Manassas has all hooves down, but he died in battle.