So, yesterday I talked about how horses don't get heart disease. What do old horses generally die of?
Obviously, in fiction, you don't have to be specific. "Old age" is as good as anything. And, depending on your culture, horses may not get to die of old age anyway - even growing up in England I saw old horses sent to the "hunt" - to be fed to the hounds. That practice isn't as common as it was, but...
But let's say your hero has put his beloved steed out to pasture behind his castle to live out his days? What's most likely to carry the old equine off.
1. Getting so arthritic he can't move or lies down and can't get up. Horses, like many large quadrupeds, will actually die from pressure on the internal organs if they stay down for extended periods of time. In modern times, sick horses are sometimes put in a sling to prevent that from happening. This would likely be a common fate of war horses, especially destriers, who worked hard in much the same way as modern sport horses. I've personally known about five animals this happened to. Once a horse gets to this point, the kindest thing to do is euthanasia. Sometimes you can get them up, but they often just go down again.
2. Colic. Colic, which I'll discuss separately, is the number one killer of horses of all ages. Old horses are less likely to recover.
3. Choke. I've already said that old horses lose their teeth. This makes it harder for them to chew their food. This means they're more likely to choke on it - which can easily be fatal in an animal with no gag reflex.
4. Nutrition failure. Best way I can put it. Between their teeth disappearing and their digestive system aging, old horses sometimes just become malnourished and starve to death. With modern care there's no excuse for this to happen - we now have special feed designed for ancient horses and there's ways you can make their life easier without teeth. But at lower tech levels, this would have happened all the time.
5. Liver or kidney failure. The liver is often the organ that goes first in old horses because their diet - and the supplements we give them - puts quite a bit of strain on it.
6. Cancer. Horses get cancers of various kinds just as much as any other species. Grey horses have a particular problem. The greying process is not a lack of pigmentation in the hair so much as all the pigmentation being concentrated in the skin. This results in a high incidence of "grey horse melanoma" - skin cancer. In most cases, grey horse melanoma just results in unsightly lumps, most commonly around the tail head, but it can spread into internal organs.
So, there you have it - a few problems your old nag might have. Note that the equine lifespan has increased dramatically in the last twenty years or so, with more and more horses surviving into their thirties.
In his twenties in this shot, Cowboy is now 35 years old and still teaching little kids to ride.