Approximate force required in pounds per square inch to pull a moldboard plow at 2 mph in normal conditions are:
sandy soil 2.5 PSI
corn stubble 3 PSI
wheat stubble 4 PSI
blue grass sod 6 PSI
clover sod 7 PSI
clay soil 8 PSI
prairie sod 15 PSI
virgin soil 15 PSI
gumbo 20 PSI (Source)
One of my coffee drinking buddies tells me that they used a two-mule team to pull a single bottom, 14" plow six inches deep and used three mules to pull two 14" bottom plows.
Figuring clay soil and the two mule team, 14" * 6" * 8 PSI = 672 pounds pulling force or 330 pounds on the draw-bar per mule. And the mules would do that all day long.
Figuring clay soil and the three mule team, 2*14" * 6" * 8 PSI = 1344 pounds
pulling force or 450 pounds per mule.
Another source claims draft horses can pull six times their weight when it is a carriage (pavement). It must be clear that this is not the tension they are putting on the draw-bar but the weight of the carriage.
A third source claims three-to-five times a draft horse's weight is a ball-park figure but highly dependent on surface and how fast you want to pull. Again, this is not the tension in the drawbar but the weight of the wagon + passengers + freight.
Stage coaches weighed about 2500 pounds and could carry nine people inside + six on top and freight. A more typical load might be six inside, two driving and fifty pounds of freight per passenger. At a buck-fifty per person, that is 2500 + 1200 in passengers + 400 in freight for a gross of 4100lbs. Teams of four or six horses would pull them at five miles per hour and get changed out every four-to-six hours. The roads were terrible.
If you can believe what you read on the internet, a horse bred for pulling can cart 1X-to-4X it weight provided it is pulling a smooth rolling cart along graded dirt roads or across dry, cut hay-fields. It can pull them at walking speed for as long as you want to sit on the seat and drive them.
One consideration is that hayfield and firewood cartage involves rest periods for loading and the return trip is deadhead, i.e., no payload.
That is a tremendous multiplier. A horse might easily pack 20% of its weight and can pull 2X. That is ten times as much payload!
Greatest invention since the wheel. Oh! Wait a minute.....