Japanese-bred racehorse is assumed to have suffered a heart attack in his stall after "The Race That Stops a Nation."
The horse swerved into the betting favorite at the start, then held off two others, including California Chrome, in a photo finish.
A guide to horse racing's biggest event, which starts Friday.
While not completely writing off a run at the 2018 Olympics, the five-time Olympian seems ready to trade in his skis for a stable.
Horse won 16 races in a row and nearly $10 million in winnings.
Buckingham Palace said positive test came from tainted feed.
British track sees close finish joined by a feathered friend.
He blamed the the outburst on emotions, adding, "I promise you, I'll do better."
He calls it unfair to let horses, like Tonalist, who did not run in the Kentucky Derby or the Preakness, to run at Belmont.
California Chrome is looking to be the first horse since 1978 to win the Triple Crown.