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BL Additional 10293 Lancelot du Lac France 054r 1316
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Entrée de l’empereur Charles IV à Saint-Denis, Grandes Chroniques de France, enluminées par Jean Fouquet, Tours, vers 1455-1460
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Sebastiaen Vrancx, Crossing the Red Sea, circa 1597 – 1600.
Medieval roads were not universally cambered and metaled and medieval carts did not have springs/suspension (See Driving?). However, horses have four hoof drive.
Pack animals were common for goods transport. Litters saw use carried by man and beast. Not everyone knew how to ride. Even on a calm horse, a long journey may have been more than someone wanted to take on.
Enter the litter. Typically these are seen carrying women and older men. They may be placed on a pack saddle or a riding saddle. They may be carried by two horses through the use of shafts.
The written record
I have have found no horsemanship specific treatises that address packing or driving except in reference to horse type. There are a few war accounts that reference baggage trains having packs (sumpters) as well as wagons. I have none specifically about litters.
From "A Description of the City of London" by Fitz-Stephens (1170):
Next packhorses, with robust and powerful legs.
From The Battle of Tannenberg or Grunwald by Duglosz (1410; trans Mikos, 1999):
They found in the Teutonic camp several heavy wagons loaded with fetters and chains which the Teutonic knights had brought to shackle the Polish prisoners, since they had promised themselves a sure victory, without considering God’s intervention and without thinking about the battle, but only about a triumph.
They also found other wagons full of pine fire-brands soaked with tallow and tar, also arrows greased with tallow and tar, with which they were going to chase the defeated and escaping soldiers.
From "The Chronicles of Enguerrand de Monstrelet" describing the Battle of Agincourt (1415; trans Johnes, 1840):
Great fires were this night lighted near to the banner under which each person was to fight; but although the French were full one hundred and fifty thousand strong, with a prodigious number of wagon’s and carts, containing cannon and all other military stores, they had but little music to cheer their spirits; and it was remarked, with surprise, that scarcely any of their horses neighed during the night, which was considered by many as a bad omen.
During the heat of the combat, when the English had gained the upper hand and made several prisoners, news was brought to king Henry that the French were attacking his rear, and had already captured the greater part of his baggage and sumpter-horses. This was indeed true, for Robinet de Bournouville, Rifart de Clamasse, Ysambart d’Agincourt, and some other men-at-arms, with about six hundred peasants, had fallen upon and taken great part of the king’s baggage and a number of horses, while the guard was occupied in the battle.
The Compleat Horseman Gervase Markham 1614 (Discourse on Horsemanship originally published 1593):
If you will choose a Horse for Portage, that is for the Pack or Hampers, choose him that is exceeding strong of body and limbs, but not tall, with a broad back, out ribs, full shoulders and thick withers, for if he be thing in that part, you shall hardly keep his back from galling. Be sure he takes a strong stride with his feet, for their pace being neither trot nor amble, but only a foot pace, he which takes the largest strides goes at the most ease and rides his ground fastest.