"From what I read, it appears the mustangs here in the U.S. Are feral. Question though: does it matter? We humans still put them here. Aren't we therefore responsible for what happens to them?"
In the US, almost no animal lives free from human interference. Bighorn sheep used to swap back and forth between the Catalina Mtns north of Tucson and the Tuscon Mtns to the west, but now an interstate and tens of thousands of houses lie between. The bighorn sheep population doesn't have the ability to move between mountain ranges like they used to do - and maybe needed to do in order to survive. Does that mean we have an ethical requirement to maintain them?
Or is "natural selection" always at work, and an ethical part of the total system? We have eradication programs for "invasive species" of plants, but the only thing truly invasive about them is they had some form of human help to get here. Once here, they out competed the "native species", which may have out competed the species found here 200 years ago.
I view mustangs as an invasive species, one that can persist because humans prevent the predators from increasing, or because humans worry about them starving to death. "Natural" would be to ignore them, and whatever gets destroyed as the herds are naturally cut down by disease and starvation and predation (which can mean an animal being eaten alive).
This is real nature: Pictures show buffalo sent FLYING when elephant digs in his tusks in | Daily Mail Online Motherly pride: Brave lioness takes on deadly crocodile to allow her cubs to cross a river in safety | Daily Mail Online
When I see a heavy coyote around here, it means he's been able to eat more because of human neighborhoods. The scat I see in the desert indicates many find it hard to fill their bellies. The deer I saw working a deer check station in the deserts of Utah many years ago probably had 1-2% body fat. I remember once looking at a hunter and exploding, "You plan to EAT that?", pointing at a German Shepherd sized deer who should have been shot to put her out of her misery. The guy heemed and hawed, but NO ONE would willing eat a doe that starved! Coyotes would, but coyotes can't buy hamburger...
My ethical obligations don't go as far as many people want them to go. If it doesn't involve my immediate family (and even if it does), my control over what happens or how it came to be is virtually nil. If I have nothing to do with how X came to be, then why do I have a moral obligation to fix X? There are a LOT of ills in the world, and I'm hard pressed to deal with the problems in my own family. I reject the idea that I have a responsibility to fix everyone else's, when I often cannot fix my own!
But if I am responsible, then my solution is to hunt them like any other species we consider invasive or harmful to others. I find holding pens obscene, and am not sure why I need to pay more in taxes to hire a ranch to keep a few hundred or thousand horses.