Okay, now that I'm on an actual computer that won't die every 2 minutes...
Even back in the "olden" days, they had the machines to cultivate, harvest, and transport hay. Not in nearly as much bulk as we can do now, but it was done. Of course, in some areas it was more complicated because the terrain wasn't good enough to provide ample grass to bale hay. There were a great many horses in those days that lived off of scrub brush and yucca plants and whatever oats or corn or barley their owner could buy at the local mercantile store.
All these pictures were taken in the area surrounding my home town (where I still live).
And, as Allison and Bsms pointed out, the horses in those days weren't the fat slick shiny things we're used to seeing now. The old horses were much more rangey. Not as much muscle mass, big bones and feet, not real tall, but a heart the size of Texas and guts 20 miles long LOL.
What the average range mustang looks like is more similar in appearance to the cow horses back in those days.
Sure, there were some nicer looking horses, but many of them were scrubby looking little things.