Kauto Star's trainer Paul Nicholls led the tributes on Twitter.
"RIP my friend you were a true Legend. Once in a lifetime," he tweeted, and attached a picture of the two of them.
The Jockey Club said: "Deeply saddened to hear the news about Kauto Star, so many amazing memories of a true champion and incredible athlete."
Sky Sports News and At The Races presenter Alex Hammond tweeted: "So sad to hear of the death of Kauto Star - a true legend."
And racing presenter Matt Chapman said: "Gutted about the passing @PFNicholls Kauto Star. Gutted. Sometimes best to say no more than that #kautothelegend."
The Daily Express' political correspondent Martyn Brown did not bring any luck for Kauto Star when he went to watch the horse.
He tweeted: "Although I was a bad omen when watching him run in the flesh, 2 runs & 2 defeats, Kauto Star was the greatest jumps horse in my lifetime."
The Irish Daily Star horse racing correspondent Ian Gaughran simply tweeted: "Kauto Star WAS Christmas!"
Jockey Sam Twiston-Davies tweeted: "Very sad to see the legend Kauto Star has gone to heaven. Such a great ambassador for the sport of jump racing and a true champ! #RIPKauto."
Tom Scudamore tweeted: "Kauto Star was the best horse I've seen and ridden against. He was good enough to win at any distance from Grade One races over two miles or a Grade One over three and a quarter he was simply brilliant.
"From his two Cheltenham Gold Cups to his five wins in the King George VI Chases, he was a star and the best horse since Arkle.
"He won four King Georges in a row then when he came third in 2010 everyone thought that was it, but he came back the next year and won. The thing was he had everything, stamina, speed, he was a freak and nothing fazed him.
"Most horses either prefer tracks to the left or tracks to the right, or they'll be better over a certain distance, or only run well on the flatter tracks. Kauto Star could run any race at any track, it didn't matter which course or which distance.
"I remember the King George in 2009, Kauto Star won by a record 36 lengths and I came second on Madison Du Berlais and I remember watching him when he past and thinking 'I've never seen a horse go so well and run so easily', it was an amazing sight."