TOKYO (AFP) - Japanese equestrian rider Hiroshi Hoketsu, 75, is to miss out on becoming the oldest-ever Olympic athlete after his horse fell ill with a cough, officials said on Thursday.

He was set to break the 96-year-old record of Swedish shooter Oscar Swahn, who was 72 years and 280 days when he won his sixth Olympic medal at Antwerp 1920.

Hoketsu, who turned heads as the oldest athlete at London 2012, had been keen to compete at the Games in August in Rio, in what would have been his fourth Olympics.

But he had to ditch his bid after his horse, Brioni W, developed a cough, Japan Equestrian Federation spokesman Azusa Kitano told AFP.

"Unfortunately I have given up on the dream of competing in the Rio de Janeiro Olympics, because I don't want to make undue demands on my horse," Hoketsu said in a statement released by the federation.

"For now, I'd like to concentrate on helping the horse get better."

His previous mount, Whisper, competed with him at the last two Olympics, but she was euthanised in 2013 after an infection following surgery.

He had been trying to qualify for Japan's four-member equestrian team in Rio before Brioni W, a 15-year-old gelding, fell ill.

Hoketsu did not elaborate on whether he will now try to break Swahn's record at the 2020 Olympics on home ground in Tokyo, when he will be 79.

"I haven't decided anything yet about the future," he said in the statement.

An appearance at Tokyo 2020 would complete a circle for Hoketsu, whose first Olympics was in the Japanese capital in 1964, when he came in 40th in show jumping.

Forty-four years later, in Beijing, he was 34th, before placing 41st in London.