It's quite interesting, they've been attempting a contraceptive scheme on Dartmoor ponies in my home county, as there is a huge wastage crisis at the moment. It's a very difficult situation, as no one likes to see all these foals getting rounded up and sent to slaughter with little to no chance at life, but at the same time, there is no demand for these ponies any more as there used to be, and the wild population is now suffering.
They trialled a contraceptive jab in 2012 on a small number of mares, and it seemed to work really well, but they're struggling to get the land to run it on more mares. It's a constant battle, because no one really claims ownership for the ponies, as they are on common land, so no one really wants to pay for them - for the government, it's cheaper for them to go to slaughter.
There are a few schemes working tirelessly to try and make life better for them though, either through contraception, or making people more aware of what brilliant kids ponies these guys make - just the right combination of sweet nature, and devil-brilliance to keep a kid on its toes
Either way, I guess it's comparable to your mustangs - something needs to be done, and, as harsh as it may seem, it might be culling some of the significant stallions, then reducing the number of breeding colts coming through and claiming herds. BBC News - Dartmoor pony contraceptive scheme at risk
Dartmoor ponies on Dartmoor - they used to be used to carry convicts to Dartmoor prison, which was unreachable by road, and supposedly impossible to break out of, as you would die before you reached civilisation - how bada** is that?!