I do agree that training a horse to be a good trail horse is its own discipline. Being a “trail horse” isn’t a default for every horse that is lame and no longer usable as ______(enter other discipline here).
To the OP, I’d also ask yourself what kind of trail riding do you want to do? Are you aiming for Tevis-type of rides, or do you like to just piddle around the pasture? What kinds of terrain/obstacles/situations will you be in?
I also agree that a lot of the horse’s temperament on the trails depends on the rider’s temperament. If the rider is nervous or scared, the horse is likely to be so also.
Ditto the suggestions of not just looking at ads, but getting in with riding groups and seeing what you can find. Good trail horses are so often sold by word of mouth, not via “sales barns” like you’d find in other disciplines.
I didn’t realize it was so hard to find a good trail horse, and it makes me appreciate my horse even more! Although I give credit to his good brain inside his big ugly mustang head. He started as a pack horse and then I bought him have been doing his u/s training ever since. We ride alone, with others, in front or behind, go through single-track trails in the forest, on the sides of mountains. We experience hikers, bikers, dogs, deer, coyotes, and just the other day a bear! We go through creeks, cross bridges, up hills, down hills, over logs. We go english, western, or bareback. Sometimes bitless, sometimes not. Sometimes we only walk, sometimes we wtc, sometimes I loop my reins over the saddle horn and gallop up the trail with no hands. Our one issue is getting hooked on mares and so I’m trying to work on that by going out with mares more.
I guess I’d say also that a good trail horse should be valued just as a good horse of another discipline. You shouldn’t expect to plop down $500 for a good horse from a CL ad…highly unlikely that it would work out the way you’d hope.