The 15-year-old enjoyed a glittering career with the champion trainer after being bought from France as a four-year-old, most notably winning the King George VI Chase at Kempton five times and becoming the first horse to regain the Cheltenham Gold Cup.
He retired from racing after pulling up in the 2012 renewal of the blue riband, having won 23 of his 41 races and over £2.3million in win and place prizemoney. Later that year Kauto Star went on to pursue a career in dressage with Laura Collett, a decision which led to a breakdown in relations between Nicholls and owner Clive Smith.
Nicholls revealed he only learned of Kauto Star’s injuries and euthanisation yesterday.
“Laura Collett kindly called me before there was a press release this morning. That’s the first I knew about it, although I understand the accident happened nearly a week ago,” the champion trainer told Sky Sports News.
“It’s obviously a very sad day and very sad news to take on board. I’m obviously mortified.
“He was like my best mate, really. I saw him every day and he was a great horse in every way temperament-wise. When he left, it was obviously a big hole we had to fill in everybody’s lives. He’d been so good for racing and so good for everybody.
“When something like this happens it’s awfully sad, but sometimes things are unavoidable. It hasn’t really sunk in, to be honest. Everyone is very upset. It’s happened and we’ve all got to get on.
“He was very sharp, not always easy to deal with and he had his own way of doing things, but he was just a brilliant horse and from day one he was always going to be very classy.”
Kauto Star looked as though his best days could be behind him after pulling up in the 2011 Punchestown Gold Cup, but famously roared back later that year to win a fourth Betfair Chase at Haydock and his fifth King George VI Chase, days Nicholls admits were particularly special.
He said: “He won 16 Grade Ones. I’ve been very lucky to have trained some incredible horses, but I’ve always said he’s once in a lifetime. To be able to win from two miles, to two-and-a-half and three miles plus, he was awesome.
“Even after he was written off, to come back and win his fourth Betfair Chase and a fifth King George said everything about him. He was just an amazing horse.
“Winning the Gold Cup was brilliant, but the two days that really stand out for me are the day at Haydock and the day at Kempton.”
The Ditcheat handler believes his pride and joy may have won a third Cheltenham Gold Cup in his final year had his preparation been smoother. “We never stopped learning about him and I think we had him at his best in the very last year he ran,” said Nicholls.
“It was such a tragedy that he fell schooling before going to try to win a third Gold Cup as he was probably in the form of his life. That wasn’t to be. He lived on the edge a little bit, he was one of those sort of horses.”
Smith explained Kauto Star had suffered what appeared to be a minor injury last week, but his condition deteriorated over the weekend.
He said in a statement: “I am devastated. He had been turned out in Laura’s paddock, as has been the case normally with him at this time for years, and he looks to have jumped something, and perhaps stumbled, we’re not really sure, but he injured himself – and it became obvious it was serious.
“The vets at Valley Equine Hospital (in Lambourn) did all they could, but it became increasingly apparent the injuries were too serious and that it was in the horse’s interests to be put down.
“The onset of secondary problems, pneumonia and laminitis, as a result of the horse being unable to put his head down and the increase of toxins in his body after standing stationary for so long, highlight the challenges faced when treating serious injuries in horses.
“I have to say Hattie Lawrence and the vets at Valley Equine Hospital did all they could and more. Kauto was kept comfortable throughout and not in pain when the decision to put him down was taken.”