England rugby star Mike Tindall has joined his royal wife Zara with a stroke of luck in the equine stakes.

The Queen's eldest granddaughter labelled her husband an "idiot" after he bid 12,000 pounds ($A18,215) for form-less Monbeg Dude at auction "without meaning to win".

"I knew nothing about him, hadn't checked the brochure and had my back to the ring," Tindall, the 34-year-old former England captain and centre, told British tabloid the Daily Star.

However, Tindall was wearing a smile after his new nag emerged the shock winner of the Welsh Grand National at the weekend.

The result has seen Dude's value rocket to an estimated 200,000 pounds ($A303,580).

But there are no plans to sell the ride, with eventing Olympic medallist Zara to be recruited for training.

"Zara will take Monbeg Dude over the showjumping poles to improve his technique," Tindall said.

"The whole thing is just ridiculous. We are beginning to think we might do better than the Welsh Grand National."

The tale is sure to impress the Queen, 86, a lover of horses and racing.