People get caught up in the 20% thing because they don't read all the way into it. No reason that horse couldnt easily carry 300 o 400 lbs at a putz around speed on easy terrain with a bit of conditioning. Look at the normal weights for ponyies and horses in military service.
The 20 percent issue was based on cavalry studies that found a horse loaded to 20% could carry that weight all day every day indefinetly with no ill efffects.
Whereas horses that were loaded to30% would wear down and need rest after a couple weeks. Somewhere along the lines the first 20% sentence was read and now it gets repeated as gospel on what a pasture pals max load is.
Remember that 20% was for a horse road all day every day for weeks and months at a time not for a couple hours aroud the block. I only have two legs and am old an out of shape and 20% wouldnt even phase me. WHen I was n shape I could carry my body weight on patrols several miles at a time