Go Native suffered a fatal injury when breaking a leg on the gallops on Friday morning, said trainer Noel Meade.
The nine-year-old was a triple 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."
He added: "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."
31 July 2015Warrior books Ebor ticket
Muthmir makes amends
Hughes at his best on Bay
Pacy Muthmir rules in King George
Mile proves golden for Beloved
Malabar stamps her class
Dettori guides Dubday to landmark
Breizh bids for Galway glory
Charlton so Close to historic win
Pharoah Phan Phestival good to go
Diamonds ready to sparkle in Nassau
Jack's the lad in Galway Hurdle
Gosden salutes 'magical' Sir Peter
Laviniad gets up late in Corrib
Simple for Beckett and Bentley
Orange pips Goodwood rivals
Shalaa dazzles in Richmond
Relief for Hughes as Gibeon strikes
Cover up for Goodwood defence
30 July 2015Clown gets serious at Galway
Eddery aiming for Mile high
Fox finale for Hughes
Ivory unveils strong Stewards' team
Treacy sent for scans
Stars on show in Nassau
Lynam 'proud' of Sir Peter alliance
Tributes pour in for O'Sullevan
It's Shanahan's turn at Galway
Sir Peter O'Sullevan dies, aged 97
Kachy not for catching in Molecomb
No Sussex regrets for O'Brien
Solow flying high at Goodwood
Highland reels in Gordon rivals
Shalaa in top trim for Richmond
Dunlop excited by Trip to Goodwood
29 July 2015Fortune favours Twiston-Davies
Peacock pair vie for Galway glory
Moviesta gets King George go-ahead
Karakontie has Deauville date
Dog delivers for Mullins and Walsh
Tint proves Topaz jewel
Maguire back in the saddle
Blue back with a bang
Tough Toormore takes Lennox glory
All roads lead to Arc for Postponed
Logan shines for Kirby
Frankie grabs Gold at Goodwood
So seeking Sussex success
28 July 2015King targeting Molecomb reign
Mullins hoping Bob is back
Wayman lands BHA role
Medrano eyeing Gordon glory
Options open for Force
Group race aim for Nisser
Modem takes control
Solitaire shines in Weld double
Bach in business
Vastonea out to sparkle in Topaz
Tanzeel unlikely for Cup date
Birch bids for Vintage victory
Toor targets Lennox triumph
Gleneagles misses Solow clash
Charlton eager for Golden Test