An old way to do work.
James Watt wanted people to know how to sell his steam engines so he compared them to horses, because they are very strong and efficiently. He did a lot of observations so he knew how much a horse can carry and how long it takes them how long.
This horse is at a coal station and he is picking it up so he can get it to the next place.
Horse With a Cart
Horse and cart, carrying coal and other items.
Horse carrying coal in the Military.