The phrase "wild horses" refers to horses that are not tame or domesticated. They run free in the wild and are not controlled by humans. Full definition
A few days later, the horse returned, leading an entire herd of wild horses. (mymommymanual.com)
The footage shown at the Game Awards did prove to present some of this feeling with enemies in the forest and even wild horses roaming the field, but what about people. (gameluster.com)
They thought they were studying the last wild horses. (sciencedaily.com)