Horses aren't simply vehicles they are living breathing animals with a whole host of issues of their own that can complicate the convenience they otherwise provide to those who use them. Roll on this table once a day for a every 12 or so horses in a party, randomy chose one of the beasts to be afflicted by the conditions on the chart if a roll so indicates something is wrong. DMs should certainly allow saves and skillful play to avoid some of the repercussions of the situations listed.
Horse Travel Hazard Table (roll 1d100)
1 or less.....Horse Up and Dies on You
2-3............Horse Breaks Leg. Horse will not be able to travel, work or fight for 2-12 months. Each day horse takes 1d6 -2 damage (0,-1 means no damage to horse)
4-6............Horse Goes Lame. Horse will not be able to travel, work or fight for 2-7 days
7-9............Foaming at the mouth- a stressed and/or dehydrated and fatigued animal, should rest remainder of day
10-13........Crack a Hoof. Can't be ridden or carry load for 2-5 days,move 1/4 rate
14-16........Throws a shoe. Move rate reduced to 1/2 if mounted
17-18........Dirt/Stones in Hoof. Move reduced to rate 2/3.
19-20.......Agitated- something has agitated the horse. Next horse reaction roll at -2. Horse will whinnie and be troublesome, double random encounter chances.
21-22.......Burrs/Biting Flies- if not resolved quickly there is 50% need to roll on Horse Reaction Table
23............Eats Poisonous Plant- Save vs poison or 1-2: foaming, 3-4: Lame, 5-6 Horse Dies
24-25.......Horse wanders- horse doesn't go where you want it to all the time, only get 2/3 movement rate in direction you wish to travel.
26 or more...all is well
-10 for each day horse is pushed
-5 if in steep hills
-10 if in swamp
-15 if in mountains
-20 if in jungle
-5 if horse if encumbered
+10 if horse is well rested/watered
This table certainly travels into the realm of sim. If DMs want a little bit of realism but not to over do the sim aspect of play an entry on random wilderness encounter checks could certainly include an entry requiring one to check this table to remind players horses are living creatures with needs and vulnerabilities of their own.