The Sumpter Horse is one of the worst Horses in the game and the cheapest horse available. Unless you just started the game, or you want to give a cheap mount to one of your companions, you won't want to use this horse in battle.
The only combat advantage this horse has is that it is slightly less fragile than a Saddle Horse, but this is only a minor advantage. At level one riding skill, a Sumpter Horse is just barely fast enough to allow for the use of a couched lance if the slope of the terrain is in its favour.
The best use of this horse is stacking them in your inventory to improve Party Speed. Note that the party speed is base value + mean value of average unit speed, so stacking them may decrease party speed in some cases.
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