Some ranchers/cowboys caught horses off the range and started riding them in the spring and turned them back to the range in the fall when they were out of a job. Then just caught more the next year.
And some of those horses were moved around with the cow herd. Where cattle can live, horses can live. Being herd animals, some outfits ran a bell mare with the remuda, knowing that she would help in getting the rest in when the wrangler went out in the morning. Wrangler kept a couple in, either high lined or hobbled, so there would be something to bring in the day horses with.
And the worst "bunch quitters," if they were worth anything, were kept hobbled until the hands could tell they'd buddy'd up with the rest.