It would be skeletal remains between 500 and 10,000 years old, the timeframe when, supposedly, there were no horses in North America.
The wild horse narrative would fall to the ground.
One such discovery was the subject of an episode several years ago of the now-defunct Mustang Matters radio program hosted by America Matters Media. The remains were found during road construction in Nevada and were quickly disappeared.
Classifying horses as an indigenous species would change many things, especially on western rangelands.