Pretty much any of the mid-to-large size dogs can make good trail dogs. Stay away from the lap-dog types, unless you want to take them with you on the saddle (not my idea of a trail dog). My favorite trail dogs have been Australian Cattle Dogs, also known as Heelers (blue or red), but over the years I have had a number of breeds. I find that some larger dogs don't have the stamina for long trails, however my sister once had a Doberman/Great Dane cross (looked like a giant Greyhound) that was an excellent trail dog and camp dog for multi-day pack trips. German Shepherds make great trail dogs. Personally, I prefer muts to registered dogs, because they tend to be healthier and heartier. They also, in my experience, have fewer psychological problems and tend to be more calm on the trail. Incidentally, Heelers are not strictly a breed as much as a type. There is no registry for heelers that I know of. They are essentially a breed that grew out of the needs of the cattle industry. If they needed a dog that barked more, they bred it to a barking breed. If they needed more aggression, they bred in an aggressive strain, etc. They have never been bred and inbred as breeds such as Labs and shepherds have. Also, Heelers are among the top five breeds for intelligence and are the longest lived of all dog breeds. I believe the longest lived Heeler on record was 27 years. They regularly live into their late teens. Good healthy dogs and good camp dogs. Heelers tend to bond to one person. My heeler loves my wife and she can teach him to do any number of tricks, roll-over, play dead, etc., but when I head for my truck, there is no force she can exert that will keep him from coming with me. There is no doubt whom he regards as his true master.
Not so good with small kids sometimes, as they tend to want to herd them and keep them with the herd. They will sometimes nip a child to keep them in line. They are also very protective of "their" children and will nip neighbor kids to keep them away. However, they get along great with other people, horses, other dogs, and are easily trained to obey voice commands, even from horseback. My dog will stay near the horse at my command, which is important when riding near roads, he will come when I call and stay near when other dogs or horses are near, he will sit at my command, as we prepare to cross roads, etc. If you want to keep your dog alive for many years of trail riding, he must be taught obedience to at least these commands from the ground, before he goes out with you on the trail. You want to be able to control him when there is vehicular traffic and when around other animals.
My advice? Go to the pound and pick out a nice-looking mut. You'll both be happy you did.
Good luck in your decision.