The horse of Brazilian equestrian rider has returned a doping test during the Olympic Equestrian competition last month.
The International Equestrian Federation says Rufus the horse of Rodrigo Pessoa tested positive for nonivamide, a banned substance known for its hypersensitising and pain relief properties.
Pessoa and Rufus finished fifth in the individual showjumping final in Hong Kong.
The FEI says the test was the sixth and final positive doping case to come out of the Olympic equestrian competition in Hong Kong.
Four riders, including Norwegian bronze medallist Tony Andre Hansen, were suspended from the Olympic Games after their horses returned positive tests for capsaicin, a chilli derivative banned for its hypersensitising and pain-relieving properties.
The FEI says it will make a final decision next month on whether the Norwegian jumping team would be stripped of their bronze medal.