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."
2 September 2014ARC "committed" to Newcastle plan
Panther primed for Leger mission
Noble on Champion Mission
Danzeno to enter big league
Story set for Dubai return
Ruler in line for Foy return
BHA turn down Newcastle bid
Towcester not "turning its back"
Demuro booked for Moon Arc prep
Chicquita nears return
Murphy excited by Purse
Surprise Matron aim for Tapestry
Maguire nearing racecourse return
Estimate among bumper Cup entry
Maguire hails 'fantastic' Peddlers
Balding staying grounded with Park
Culloty joins Leger Legends
Regard among September possibles
Cox takes Haydock plunge with Love
Paddy gunning for Chepstow
Athenian has sales date
No hurry for Cape call
Mirza on Abbaye trail
1 September 2014McCain salutes retiring Peddlers
Seven Towcester meetings go to ARC
Further checks for Condon
Due set to miss Sprint date
Condon 'stable' after Cork fall
Queen rules supreme for Smith
River eases to debut success
Carla tops it up to four
Atzeni strikes on Cappella
Ocean team want Haydock rain
Big time beckons for Orange
Palmer aims high with Aktabantay
Hills ponders Dutch gameplan
31 August 2014Atzeni strikes on Cappella
Imminent return Likely
Logic on comeback trail
Dan edges closer to Mile high
Keane error costly for Unrequited
Notarfbad makes all in Mildmay
Orange enjoys Big win
Fabre strikes with Fintry
Akta in late Solario swoop
Icon takes starring role
Pearl fires home in Bullet
Shortall suffers injury setback
Tent team consider chasing
30 August 2014Keane error costly for Unrequited
Shepherd's sent to Tower
Time will tell for Charlton
Stack filly Alive to chance
Sansevero sent to Tower
Palmer plots Solario success
Fabre has high hopes for Fintry
Belardo team keen on Champagne
Rigueur targets Chester consolation
Native in hunt for Bullet gains
Arc still target for Taghrooda