small hardy range horse of the western plains descended from horses brought by the Spanish
"The cowboy herded a herd of mustangs across the dusty western plains to find fresh grazing land."
A small, hardy, naturalized (feral) horse of the North American west.
"The cowboy herded a band of wild mustangs across the dry plains of the North American west."
In plain English: A mustang is an untamed horse that roams freely on public lands without any owner.
"The mustang ran freely across the vast plains without any rider."
Usage: Use this term to refer specifically to feral horses descended from domestic stock in the western United States and Mexico. Do not use it as a synonym for any wild or untamed animal outside of equine species.
The word mustang comes from the Mexican Spanish term mestengo, which originally meant a feral animal. It entered English to describe wild horses descended from domesticated ones that had escaped into the open range.