A smartly-bred yearling died and six others were injured when they took fright at a military plane flying over their field, their owner his claimed.
The full-sister to Group Three winner Angel's Hideaway suffered unsurvivable cuts to her legs during the incident in Tewkesbury, Gloucestershire.
She was in a field with companions at Natton House Thoroughbreds when a military aircraft, believed to be from the US Air Force, passed overhead.
On hearing the noise, the group of youngsters set off at speed and jumped several fences, which caused their injuries.
Four staff members at the stud farm, which produced Cheltenham Festival fourth Navajo Pass, found them distressed in the field the following morning.
Part-owner Chris Dudfield said: "It was very upsetting to see these lovely young horses with many cuts on them.
"Sadly one, a full-sister to Angel's Hideaway, had to be put down. She was a valuable horse for the sales.
"We think from local reports it was a plane which startled them.
"They had been in that field for months with no problems at all."
It is thought that the CV-22 Osprey, based at RAF Mildenhall, was the aircraft flying over on March 31.
Gloucestershire Live reported the US Air Force plane, from the 352nd Special Operations Wing, was seen above surrounding areas including Cheltenham and the Cotswolds.
The yearlings have been kept inside since the incident and they have recently had their stitches removed.
Dudfield, whose business has 29 mares in foal including one to the world's joint top-rated racehorse of 2018, Cracksman, said: "Others that were in the field were sisters to Group Three winners Great Scot and Fly On The Night, a filly we bred here.
"It is a challenging time at the moment as it is and not knowing when the sales are going to be held.
"The costs of this run into five figures."
In 2019, the farm sold a Kayf Tara colt for £28,000 at the Goffs UK January Sale.
The buyer was Peter Molony of Rathmore Stud, who went to €100,000 for 2020 Cheltenham Festival winner Honeysuckle two years ago.
A spokesman for the US Air Force based at RAF Mildenhall said: "The Ministry of Defence has not been made aware of any incidents or complaints on or about March 31, 2020, in Gloucestershire or the Cotswolds involving low flying."
