Context: My Undead Lord Cleric's current Corpse Companion is a fast zombie heavy horse.
So, my GM and I disagree on a key aspect of riding an undead horse. He thinks that because riding a living horse is about as tiring as jogging, it should be the same on the undead horse. I agree that riding a living horse is tiring, but the work the jockey has to put into riding the living horse is a) to keep from tiring the horse out prematurely and b) to prevent the rider's weight injuring the horse, neither of which should apply to the abomination against nature my character is riding.
Oddly enough, I believe the rules are already in my favor, because normal riding doesn't appear to take actions and therefore shouldn't be tiring at all, allowing me to go on only obeying the rules for the need for sleep; but it's very rare I do anything with mounts in my own games and I'm pretty rusty.
(Because I can be a bit of a rules-lawyer, here are some specific things this question is not about: Rule Zero, whether a fast zombie can be a corpse companion, whether an undead horse can count as "war-trained".)
Best Answer
The horse does not get tired. It doesn't necessarily mean that YOU don't get tired. You're still going be bouncing up and down (unless you've ALSO fused with the horse). You still have to guide the horse (more so, in this case, because your horse can't think). You have to stay ON the horse. It won't try to throw you, but it's still moving very fast. It's not just dragging you along in a carriage, so why wouldn't you get saddle-sore?