A leading Dutch dressage rider has pulled out of the Olympic Games after her horse became ill at Rio.

Adelinde Cornelissen quit the Games just minutes into the dressage event after her beloved horse Parzival fell ill the night before.

Following an individual silver and team bronze win at London 2012, Cornelissen noticed her horse in a state of distress.

"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 wrote on her Facebook page.

After a batch of tests and fluids, the swelling in Parzival's head decreased and the horse was cleared for competition.

However, after making only a few moves into the competition, the rider decided to quit to protect her horse.

Parzival is 19 years old, meaning it's likely Rio will be his final Olympics.