Those mustangs are rounded up several times a year because of swelling populations; now some of them are being put to work to patrol our country's borders.
The horses are broken by cowboys, but not the cowboys you might expect. They are inmates at the Department of Corrections prison in Florence.
The cowboys are hardened criminals, who while rehabilitating themselves are training these wild horses to work for the Border Patrol.
"The less you can put pressure on his head, the better he's gonna be when you put a bridle on him," said inmate Brian Tearse.
Breaking a horse these days is a far cry from the old cowboy movies.
"This is what really brings the horse's energy down, gentles him, lets you next to him," said Tearse.
Brian Tearse is serving a seven-year sentence for aggravated assault. When FOX 10 met with him, he was working with Drift, who used to be an American mustang living on public lands, un owned and definitely untamed. It's something that Tearse can relate to.
"Oh they are wild, just like I was when I showed up in prison; wild. I've been gentled down, and that is what we're doing with these horses," he said.
Tearse is the lead trainer in the program that turns the horses that work to protect our borders.
The horses get intense training here, like one exercise where they push barrels around; the horses are getting used to pushing people.
After two months of training, the horses are sold to the U.S. Border Patrol and go to work.
Randy Helm is with the Arizona Department of Corrections; he is in charge of the program.
"It's one of those win-wins, the horses are getting good homes, and you are giving the inmates something to do. So it really is one of those win-wins," said Randy Helm.
Starbuck, one of the horses, has only been ridden for a year. The total cost to the Border Patrol is $500. Normally they would pay about $5,000 for a horse like this.
"It's a huge cost-savings to us at a time when everybody out of the federal agencies is experiencing budget cuts... they've been great horses," said Bobbi Schad with the U.S. Border Patrol.
The program behind the razor wire and the imposing gray buildings is just really getting into gear. The Border Patrol is looking to buy about four horses a month.
The inmate-cowboys here think they can keep up with the demand, and they know they are getting a lot out of the process.
"I'm like the horses in the round pen, the Lord is working in my life and he's gentling me in this round pen, just like I'm gentling the horse," said Tearse.
The Border Patrol used to go all the way to Colorado to purchase trained mustangs. They say because of the success of the Florence program, they will now buy all their horses in Arizona.