Noel Meade's Randox Health Grand National contender A Genie In A bottle was found dead in his stable on Friday.
The seven-year-old's death is a mystery to his connections at present.
He was owned by Gigginstown House Stud and had been around 50-1 for the National but was sent off 4-1 favourite for the four-mile National Hunt Chase at Cheltenham last season where he finished fifth.
"We've no idea what happened. He was okay in the morning and dead in his box at lunchtime. It's just one of those things," said the Gigginstown's spokesman, Eddie O'Leary.
Gigginstown are still set to be well represented in the Aintree showpiece.
"Hopefully Tiger Roll will run and Alpha Des Obeaux. Thunder And Roses might run because he fell at the seventh in Fairyhouse and so had no race," O'Leary told the Irish Times.
"Valseur Lido may run too, and Road To Riches. Hopefully the place (Aintree) might help him find his mojo."
O'Leary also mooted the possibility of Rachael Blackmore joining Katie Walsh (Baie Des Iles) and Bryony Frost (Milansbar) in the National.
"There's nothing definite but Rachael is unbelievably good and we'd love to have her ride something in the race," said O'Leary.