I would be v weary of your book from the 1920s
You say yourself that horsefat is low in fat
did he try any other meat?? I am guessing that they used horse and possibly rabbit because that is what they had cheep around at the time, so as those meats are low in fat then they needed to add the carbs to get the energy, it was not an optimised diet - just optimised for horseflesh

I have been lent an old 1922 book on hunting beagles. It was written by a man who was master of the Thorpe Satchville Beagle pack for 30 years. The Thorpe Sachville Beagles are one of the oldest and most well known Beagle packs and in fact it's thought that pretty much all Beagles in the world will trace their pedigree back to hounds from this pack. According to him tried and tested diet that both fox hound and beagle huntsmen found to be best during the hunting season is in keeping with what I'm finding with my Beagle (i.e. a high carb diet). He does suggest feeding a huge variety of grains, meats and veggies in the summer when the dogs aren't working, but during the hunting season just one specific meat and carb source.

The science supports why my beagle sprints best on a high carb diet, but it didn't really explain why she had better endurance on high carbs too. Hunting beagles need to run hard all day. Feeding a high carb diet doesn't fit with the science (endurance dogs *should* do better on a high fat diet) and it doesn't fit with your wolf theory (no carbs). So is this a case of hunting dogs evolving to thrive best on the diet that people with large packs of hungry dogs to feed could afford? The author of the book was really only talking about Beagling experience over a 100 year period. But Beagles have been mentioned in history for hundreds of years. So were they selectively bred in the early years for their ability hunt well on a cheap high carb diet?

The best diet was found to be a mixture of oatmeal and horseflesh. Other meats didn't work as well. The interesting thing about horsemeat is that it's very high protein and VERY low fat. It breaks down as follows:

Oatmeal

10.84% water
13.15 % protein
6.52 % fat
67.7% carbohydrate

The amount of fat, protein and carbohydrate on a dry matter basis is...

13.15 / 89.16 * 100 = 15.42% protein
6.52 / 89.16 * 100 = 7.3% fat
67.7 / 89.16 * 100 = 75.93% carbohydrate

Horsemeat

21.39% protein
4.6% fat
72.63% water
0.99% ash

The amount of fat and protein on a dry matter basis is...

21.39 / 27.37 * 100 = 78.15% protein
4.6 / 27.37 * 100 = 16.8 % fat
0.99 / 27.37 * 100 = 3.62% ash

No matter what way you mix the oatmeal and horsemeat you can't get more than between 7% and 16% fat in the diet. Which means that there's apparantly no fuel for the dog's fat driven aerobic energy systems. There is no evidence that the Beagle has a different digestive system or different organs from other dogs. Yet despite being an endurance animal it *apparantly* manages to function best without the fuel that's known to be necessary for endurance in dog's.