Adelinde Cornelissen says she withdrew her horse Parzival from the team dressage at Rio 2016 to protect him after he contracted a fever from an insect bite.
Dutchwoman Cornelissen won silver in the individual dressage and bronze in the team event at London 2012 on Parzival but was forced to bring the routine to a halt on Wednesday.
Speaking afterwards, Cornelissen revealed Parzival had been bitten by "a spider or a mosquito" and that his body had been "full of toxics" but was cleared to compete by the team vet.
Elaborating on her comments on her Facebook page, Cornelissen wrote: "I discussed with several vets and the team coach.. The FEI [International Federation for Equestrian Sports] vets came and checked up on him around 10am. All gave green light to compete.. We decided I would give it a try.
"The fever was down, which means the toxics are out of his system. There is still a bit of swelling in the jaw, but we cooled it all the time and the swelling gets less...
"I agreed to give it a try. Knowing the story, I took it easy in the warm up and didn't want to ride full. He was ok, although he didn't feel very powerful.
"I walked a lot to save the energy.. When I entered I already felt he was giving his utmost and being the fighter he is, he never gives up... But 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."
The withdrawal means the Netherlands team will have to compete as a three-piece.