The Mustangs are an invasive species in the west and their population growth is devastating the local flora. "60 Minutes" ran an episode about it recently.
they don't sting so they're fine. Also in North America horses lived here 10kya before going extinct so at least here they're resuming an ecological role that was once occupied by their ancestors
Also x2, horses originated in North America! They came to eurasia over the bering land bridge!
So the horses that were released (and have thrived!) in north america are originally north american horses. It's part of why they do so well-- they can eat all of the native brush here. Whereas if a cow gets loose in n. america it's gonna poison itself.