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."