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."
25 October 2014Muir full of praise for Code
Hawk swoops for Aintree prize
Smaih shines for Hannon
McCoy delighted with World hero
PJA and BHA issue fraud warning
'Enigma' Seanie ready for Knockaire
Prendergast duo set Eyrefield poser
Tac takes aim at Royal-Oak double
Snoano stepping up for Trophy bid
Adelaide rules in Cox Plate
Sir Max clinches Mercury gold
Full marks for Bangor schooling
Milky Way on another planet
O'Brien targets another Trophy
Hurricane in hat for Morgiana
Buick happy as a Star is born
King ace back in the Old routine
McCoy makes Chepstow return
Botti seeking Astrelle celebration
24 October 2014Ballyalton out for the season
Hutchinson sits out weekend action
Balding sweet on Park chance
Walker hoping for Glorious victory
Times out to make Newbury headlines
Cheltenham honour leading figures
Moore keen on Adelaide chance
Stevens back for Breeders'
Johnson Houghton fined
Duke lords it at Carlisle
Southwell first for O'Brien
Fenton fined over steroids
Buywise on the money at Ludlow
Lockinge the big plan for Toormore
Stone ruled out of Old Roan
Wetherby likely for First run
Elm tops Trophy octet
23 October 2014Fenton guilty of drugs possession
Williams hoping for Max power
Harty biding his time with Lane
Clonmel aim for Baily
Jezki possible for Morgiana bow
Harrington expects Naylor to shine
Gosden a Fan of Newmarket scorer
Northern aces face off in Cumbria
Murphy plagued by shoulder injury
Dynaste charts direct Haydock route
O'Brien outlines Breeders' plans
Wetherby-bound Taquin to duck Power
High hopes for Adelaide run
Gilgamboa waits on ground
Dandino misses Cup date
22 October 2014December date for Balding memorial
Deputy outgunned at Fontwell
Whisky chasing Power spoils
Burke maps out Odeliz targets
Present sights set on Power prize
New category for all-weather champs
Moviesta has sales date
National aim for O'Neill star
Cheltenham on Ginge agenda
Lewis eyeing Dubai success
Henderson handed Ascot role
Panther given Breeders' go-ahead
Plain sailing for perfect Storm
Big Easy sets sail for Open fixture
Hall monitored for Giant Bolster
Stomachion in November frame
Hughes revels in 'sweet' success
BHA moves to address small fields
Connell turns to Heskin
Integral to race on next year