The accident happened during the qualifying rounds for the 54th CHI international horse show jumping tournament in Switzerland.
Athina Onassis' 11-year-old mare Z-Zang was injured in a fall
Athina was riding her white mare, Ad Camille Z-Zang. The horse was removed from the arena using a crane and taken to a veterinary clinic, where the decision was taken to end her suffering.
It was an hour before the competition could resume and Athina was in tears, clearly distressed.
It will be even more unsettling for the 29-year-old heiress as she injured her spine after a bad fall in 2012.
Athina Onassis and her husband Doda de Miranda are both keen equestrians
Despite inheriting half of her grandfather's billion-dollar fortune, she has largely lived a quiet life, based in Sao Paulo, the home of her husband Alvaro 'Doda' de Miranda and Florida.
He is also an equestrian and the couple spend the year training their horses or touring the world in competition.