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.
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]
[0] https://priceonomics.com/when-americans-ate-horse-meat/