British Eventing paid tribute to Iona Sclater, 15, saying she was an "exceptionally talented and dedicated young event rider".
Emergency medical staff were called to her home in the village of Abington Pigotts in Cambridgeshire at around 11.30am on Sunday, but were unable to save her life.
The teenager had recently posted a picture of herself with a horse captioned: "Find me someone who looks at me the way Swatchy does".
Hundreds of people have posted tributes on the image, with one reading: "Blessed to have known you for almost five years ... rest in peace, completely heartbroken."
In a statement, British Eventing said: "British Eventing was devastated to receive the news that one of our young members and bright stars in the sport, Iona Sclater, sadly died in a riding accident at home on Sunday 11 August.
"Long-listed for the 2017 and 2019 European Championships for Ponies, fifteen-year-old Iona was an exceptionally talented and dedicated young event rider.
"Our thoughts are with Iona's friends and family, her Dad, Charles, Mum, Hetty, and sisters Lara and Alicia, at this very difficult time."
In five seasons with British Eventing, the young rider had achieved 33 top 10 results and represented the Eastern region four times at the under-18 Regional Team championships.
She had also secured a place on the Pony European Championships long-list.