Denise Hill, 35, her husband, Jonathan, and their nine-year-old son had travelled with friends to stay at a "dude" ranch in Carbon County near the Wyoming border.
Bob Reed, Carbon County undersheriff, said the accident happened about 35 miles north of Lovell, Wyoming.
"Her foot got caught in the stirrup and the horse spooked and bolted. It was just one of those terrible accidents which can happen," he said.
Rescuers were hampered by the remote terrain and took about two hours to reach Mrs Hill, from Woodthorpe, near Chesterfield, Derbyshire, who died at the scene.
In a separate incident, a bus driver on a five-month African adventure holiday died after he became separated from his group and fell 15 metres down a mountainside, an inquest was told yesterday.
Christopher Holden, 29, from Birmingham, probably lost his footing while trying to climb down a gorge in the dark, near Ouarzazate, Morocco, last November. The city coroner, Richard Whittington, said Mr Holden's injuries were consistent with a fall and recorded a verdict of accidental death.Reuse content