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
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.
https://awionline.org/content/wild-horses-native-north-ameri...
https://returntofreedom.org/new-research-proves-yet-again-th...
That is to say, their natural predators are either not present, not present in large enough numbers, or forbidden by law to hunt them.[0]