Naomi Brown told local news outlets that her 3-year-old daughter was left traumatized by last week's incident, which made headlines in one of Britain's top-selling tabloids, the Daily Mail.
"The horses were galloping around fine at 12:30 p.m., at about 3:30 p.m. the low-flying planes were spotted in the area and at about 4:30 p.m. our pony was found in a very bad state," Brown told the newspaper.
Brown said an emergency vet was called in, but that it was too late and the animal had "twisted his gut and died before the vet could arrive."
"These aircraft were flying in accordance with the rules and limitations of the UK Low Flying System and would have been no less than 500 feet above the ground," the Air Force said in a statement.
The incident has been referred to the U.K. Defense Ministry office that handles public complaints about low-flying aircraft.
The Air Force said it would cooperate with any formal investigation into the matter.