Errant Horses

Errant horses are free roaming horses,  descendants of domesticated stock, living in wild or in racing contexts, horses that behave waywardly during a race, often running unevenly. Ferel horses, like Mustangs, form small organised bands, while adapting to wild environments, while errant in razing describes a horse lacking focus or steering properly. 

Feral Horses:  These are domesticated horses, or their offsprings that have escaped and live in a self-sustaining wild state. They are not technically wild species, but are adapted to live without human management. 

Mustangs are free roaming horses in the Western US, descended from Spanish conquistador horses.

Wayward Horses: An errant horse in racing is a wayward horse.