That was Hollywood. The horses back then were often actual ranch horses or strings of horses the were privately owned and worked in Arizona in the winter and farther north in the summer. The point is that they were not horses that had been leased for the season with no one knowing exactly what the horses had been doing immediately prior to their current job. At least in some cases today, trail ride strings of horses have been put together at the auction barn right before the season began. Certainly this is not true of all trail rides and dude ranches but it is true of many because many places do not have a way to maintain horses through the off-season.
Some businesses simply rent a corral and the horses are hauled out for rides.
This doesn't mean that the lease horse string is bad by definition, only that it is different. The last difference is in the staff. Most places in most areas are fighting to find summer staff. Gone are the cowboys from those Roy Rogers movies. No longer is the ride lead by the boys who will be playing their guitars and fiddles after dinner, if they ever were.