Per the 1e Wilderness Survival Guide:
Heavy war horse: 500 lbs. normal load, 750 lbs. max.
Light war horse: 300 lbs. 500 lbs. max.
Medium war horse: 400 lbs. 650 lbs. max.
Wild horse: 300 lbs. 600 lbs. max.
Mule: 500 lbs. 750 lbs. max.
Pony: 200 lbs. 300 lbs. max.
Any loads larger than normal = 1/2 movement penalty. Of course there’s all sorts of fatigue considerations, which typically the WSG goes into great detail about, but I’m not concerned with that here. OSRIC mentions that only 10% of horses can be trained as warhorses. This complicates things because there is no “Riding horse” listed in the WSG. There is also mention in the WSG of vehicle movement rates and loads. There, the description of mounts uses light, medium, and heavy again, but the reference to them being “war” horses is gone. A couple of thoughts: is it really necessary to have three types of warhorse? Or a wild horse and pony even? Ok, my prejudice for B/X is showing through. Still - “Do you have a pony sir?” “Erm, you mean you want a punier horse? Sure, I got some of them over here...” Anyhow.
In Labyrinth Lord, this information is found in the monsters section of the book, under Horses. Here we have something more to my liking - three types of horses.
Riding horse: normal load 300 lbs. 600 lbs. max.
War horse: 400 lbs. 800 lbs. max
Draft horse: 450 lbs. 900 lbs. max.
With the same movement penalty assumed, although not quite as explicitly spelled out.
Holmes Basic D&D has the following to say - under the entry of horses in the monsters section, four types of horses are listed: Light horse, Medium horse, Heavy War horse, and Draft Horse. Nothing is said here about weight limits, but mules are mentioned under the horse entry as being capable of carrying 350 lbs.
Cook & Marsh’s Expert D&D has more information under its entry for Horse in the monsters section, although now we are down to three types (the same used by LL): Riding horse, War horse, and Draft Horse. Loads are also the same as Labyrinth Lord.
I don’t believe S&W mentions these things anywhere, and I’m not sure about OD&D. Perhaps this information will be useful to someone devising their own set of house rules. I wonder if anyone’s already developed something at odds with these fairly close sets of stats, and how they arrived at whatever they chose to use.
I’m glad I took the time to look these up - although I don’t track encumbrance very much (unless it becomes ridiculous) with regard to my players’ characters, I’m more inclined to track it when it comes to mounts and beasts of burden. I don’t want to make it too easy for vast sums of treasure to be hauled away, and want the players giving a certain amount of attention to logistics when it comes to heading back to town with their loot, traveling long distances, and deciding what to take and what to leave behind on occasion - so the only question remaining is, exactly how much can a