Overland and Encumbrance Pt. 2 - Horses, Dogs, and Pack Animals
Horses are simple enough. They're described in the Monster Manual.
An ordinary horse will not enter a dungeon. They are 90% likely to panic at any danger - fire, sudden movement, loud noises, or taking damage. A warhorse only panics 10% of the time, and doesn't panic at sudden movement or noise. A normal pony, donkey, or mule panics 50% of the time, and a war-pony never panics. A pony, donkey, or mule will enter dungeons.
In terms of encumbrance, horses work very differently to humans. Overland, we only care if the horse is moving at normal or half rate, and the type of horse. Armor has no effect on a horse beyond its weight.
A rider encumbers a horse for 2000 GP before any of their equipment. A horse can move at full speed while under a half-speed load for 1/3 of a day and suffer no ill effects, so long as it is under its recovery load for the rest of the day. If the horse is ridden more than it is allowed, it must save vs death at the end of the day.
Carts improve a horse, pony, donkey, or mule's carrying (but not recovery) capacity by 1.5x. They encumber the horse for 1500. Wagons improve a draft horse's carrying (but not recovery) capacity by 2x. They encumber the draft horse for 3000.
DONKEY: 30 mi/day, 0/1500/4500 recovery/half speed/no movement.
MULE: 30 mi/day, 0/2000/6000 recovery/half speed/no movement.
DRAFT HORSE: 30 mi/day, 2500/4000/8000 recovery/half speed/no movement. Consumes DOUBLE FEED.
HEAVY WARHORSE: 30 mi/day, 1000/5000/7000 recovery/half speed/no movement. Consumes TRIPLE FEED.
MEDIUM WARHORSE: 40 mi/day, 800/4000/6000 recovery/half speed/no movement. Consumes DOUBLE FEED.
LIGHT WARHORSE: 60 mi/day, 600/3000/5000 recovery/half speed/no movement.
PONY: 40 mi/day, 800/2000/3000 recovery/half speed/no movement.
RIDING HORSE: 40 mi/day, 600/3000/6000 recovery/half speed/no movement.
(a late edit - i didn't mention dogs despite them being in the title!)
DOG: 30 mi/day, 500/750 half speed/no movement.
PACK DRAGONFLY: 80 mi/day, 100/1000/2500 recovery/half speed/no movement. Pack dragonflies wear themselves out after a day of flying above their recovery load, no matter what, and can't negate half speed like horses can. However, they ignore terrain. Nomads do not sell these, but some clans keep them.
MARSH LIZARD: 30 mi/day, 1000/2000/4000 swimming/half speed/no movement. Amphibious below recovery weight, swimming across lakes or rivers at half normal speed. Marsh lizards can't swim for more than a full day in a row. Ignores swampy terrain and has no trouble with river crossings. Some nomad clans sell them.
MOUNTAIN LIZARD: 20 mi/day, 4000/8000. Considers mountains as merely Rugged terrain.