Noel Meade and his team were left "devastated" after Go Native suffered a fatal injury when breaking a leg on the gallops on Friday morning.
The nine-year-old was three times a Grade One winner and had looked back to his best this season following a long absence.
Meade said: "We're all in a state of shock at the moment.
"He was just doing a very steady canter. I was stood about 100 yards away from him and all of a sudden I heard this almighty crack and he'd somehow broken his leg above the hock.
"I don't know why these things happen. He was the first one to come up the straight gallop after it had been harrowed.
"We are all completely devastated. It's a cruel blow for his owners and everyone in the yard. "
Owned by the Docado syndicate, Go Native returned from an absence of 939 days to be third on the Flat at Navan in October and went a couple of places better subsequently at Galway.
He went back over hurdles in the Morgiana Hurdle at Punchestown last weekend only to fall at the final flight when a length behind the winner, Hurricane Fly.
Referring to the Morgiana, Meade said: "You have to question whether he did something in the fall, but although he's not done any fast work, he's been hacking all week and has seemed fine.
"We had the physio check him out thoroughly earlier in the week and she was happy that he was perfect. We'll never know now.
"He had all that time off and came back as good as he ever was and for this to happen is just incredible.
"You have to be a hard man in a game like this, I tell you.
"I know there are lots of other businesses where you get knocks, but there can't be many harder games than this."
Meade's inmate won nine races from 18 starts under Rules, including the Christmas Hurdle at Kempton and the Fighting Fifth Hurdle at Newcastle three years ago.
But his finest hour came a season earlier when he claimed the Supreme Novices' Hurdle by a neck at the 2009 Cheltenham Festival.
Go Native earned over €400,000 in prize-money for his connections.
Boylesports elected to refund ante-post bets on Go Native for this year's Fighting Fifth and the Cheltenham Festival in March.
Expressing their sympathy, Newcastle racecourse tweeted: "Devastating news, sorry to hear the sad loss of Go Native @NoelMeades yard. Our thoughts are with the team @TuVa. #FightingFifth winner".