O'Neill says the horse has a swollen left front tendon, prematurely ending the horse's promising career in a shocking move.
I'll Have Another was trying become the first horse to win the Triple Crown since Affirmed in 1978 after the horse took first place in both the Kentucky Derby and the Preakness.
"It's been an incredible ride, an incredible run," O'Neill said. "It's a bummer. It's not tragic, but it's a huge disappointment."
Nassau County Off-Track Betting said that all advanced wagers will be refunded. I'll Have Another was the 4-5 favorite to win the Belmont and become the 12th Triple Crown winner and first since 1978.
"Yesterday he galloped great, but in the afternoon we noticed some loss of definition in his left front leg," O'Neill said, addressing the media outside the Belmont barns while I'll Have Another grazed nearby. "We did just an easy gallop today. I thought he looked great on the track, and then cooling out, you could tell the swelling was back."
O'Neill said he conferred with owner J. Paul Reddam and they contacted Dr. Jim Hunt, who examined the horse.
"... Immediately we got Dr. Hunt over here and he scanned him and he said it was the start of tendinitis in his left front tendon and you can give him 3-to-6 months and start back with him," O'Neill said. "It was unanimous between the Reddams and my brother and I and everyone at the barn to retire him."
O'Neill and owner Reddam stressed that this was a freakish event and was not the result of training. The horse is apparently in good enough shape to run the race, but O'Neill had reservations about the possibility of a catastrophic injury, like the one suffered by Barbaro.
"It's devastating," said Dale Romans, trainer of second favorite Dullahan. "I thought this was going to be one of the greatest races in history, and I wanted the opportunity to be part of it. But this is bigger than that. This is terrible news."
Last September, I'll Have Another came out of a losing effort in the Hopeful Stakes at Saratoga with shin problems and took the rest of the year off.
He returned to racing in February, and won the Robert Lewis Stakes at Santa Anita, putting the colt firmly on the Triple Crown trail.
O'Neill and Reddam immediately gave I'll Have Another two months off leading up to the Santa Anita Derby, which he won by a nose on April 12.
The horse becomes the 12th since Affirmed, the last Triple champion, to win the first two legs but not the Belmont.
The scratch marks the first time since Bold Venture in 1936 that the Derby and Preakness winner didn't run in the Belmont. Burgoo King skipped the race in 1932.
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