Adelinde Cornelissen shocked fans by quitting the Games mid-test after her horse Parzival contracted a fever the day before the event.
Cornelissen and Parzival, who won individual silver and team bronze together at London 2012, retired from the individual Grand Prix Wednesday after only a handful of movements.
And it's likely to have been Parzival's final Olympic performance because the Dutch warmblood gelding is 19 years old.
Cornelissen took to Facebook
and explained she pulled out of the Games over concern for Parzival's welfare.
The day before the event, she said her horse's head was swollen and Parzival had developed a fever, the exact cause of which is still unknown.
"After double checking with the vets here they concluded he was bitten by an insect or spider or some sort of animal which produces toxics," the 37-year-old Cornelissen added.
Parzival was given fluids and had X-Rays, swabs and blood tests taken and eventually his temperature fell.
Cornelissen said the Dutch team had asked to alter the starting positions of the team to give the horse another day to recover but the request was refused by the Fédération Equestre Internationale (FEI).
On the day of the event, Parzival was deemed well enough to compete by FEI vets but during the test, Cornelissen realized something was not right
"In the arena he felt totally empty and I decided not to continue. He did not deserve this," she continued.
"In order to protect him, I gave up ... My buddy, my friend, the horse that has given everything for me his whole life does not deserve this ... So I saluted and left the arena."
Second bullet found
Meanwhile, there was more trouble at the equestrian venue as a second stray bullet was found at the site in Rio de Janeiro.
Reports in Brazilian local media say that the bullet was found at the stables where some of the world's most expensive horses are being kept.
When asked about the incident, the Rio 2016 Local Organizing Committee said: "Police authorities have investigated the incident and have shown us that the incident does not affect the security of the venues."
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This is the second incident with stray bullets at the equestrian venue.
On Saturday, Mario Andrada, communication director for Rio 2016, confirmed that another bullet had been found in the media tent at the horse park.
In that case, Andrada said the bullet was a stray from a nearby community.