De Leyer, who moved to the US with his wife after escaping Nazi-occupied Germany, bought the gelding to teach children to ride, never expecting "Snowman" to become the greatest show jumper of his generation.
"I feel happy every day that he was in my life," De Leyer says from his Virginia farm in a telephone interview with CNN 42 years on from the passing of what become affectionately known as "The Cinderella Horse."
"His pictures are still in my room, he's where I sleep and he's still in my heart. He's always still with me, he'll stay in my mind my whole life and I still think of him every day. He was my best friend as well."
Now aged 88, De Leyer still rides and teaches children to ride, living up to his moniker of "The Galloping Grandfather." given to him when he represented the US at the 1983 World Championships, in the process.
But his life in the public eye remains defined by his decision to buy a placid-seeming horse for children at the affluent Knox School in Long Island
'A very special horse'
Late to an auction that day, all that was left were the remnants packed up for the slaughterhouse.
"I needed a quiet horse for the beginners," he recalls, vivid in the recall as though the events of yesterday. "I remember seeing his eyes and thinking 'this one seems nice and quiet', I'll give him a chance."
He haggled over the price, paid a sum of $80 and had him dropped off later that day.
Loved by the children he taught as well as de Leyer's own brood of eight offspring, Snowman -- his name chosen by his children as the snowflakes fell in his first winter with them -- was a placid horse who never once reacted when the kids pulled his mane or tail roughly.
"He was a very special horse," explains his former owner. "He just had a special temperament. He was always good, he always tried to please, he pleased me the most."
But he could also jump, a facet de Leyer only realized when he sold Snowman to a neighbor, who needed an easy horse to plow his land.
Snowman would jump the high fencing that separated the two properties and always return to where he felt was home.
"I sold him once but the bugger kept on coming back," says de Leyer laughing in the retelling. "He kept on turning up at my barn and I joked to the guy that clearly he didn't want to be sold. So he suggested Snowman boarded with me. Two months in he'd never paid me a penny so he just told me to keep the horse."
After that, he and Snowman were never parted but Snowman's ability to clear the fence between the neighboring properties made de Leyer think there might be more to the horse than he first thought.
De Leyer was a talented rider in his youth in Holland before the Nazi occupation put paid to any ambitions he might have had to make the Dutch Olympic team. In the unlikely guise of Snowman, he finally had the chance to take on America's finest show jumpers.
In 1958, less than two years after being bought for $80, Snowman won the then Triple Crown: The American Horse Shows Association Horse of the Year, Professional Horseman's Association Champion and Champion of Madison Square Garden's Diamond Jubilee
"I never thought he'd be a champion at Madison Square Gardens," de Leyer repeatedly says, seemingly still in disbelief at the horse's performance against the world's best show jumping horses which today change hands for as much as $12M
"I remember people laughed," he says when aficionados first laid eyes on the pairing in such illustrious competition only to realize the horse had such an ability to jump high that he would even jump over other horses.
"So the crowd just loved him. He was the very cheapest of horses doing the absolutely impossible. After Madison Square Gardens, he was a superstar, everybody knew about him."
The adulation started with articles in the New York Times and followed with TV appearances on game show "To Tell the Truth" and "The Tonight Show," on which host Johnny Carson opted to sit back to front on Snowman.
"Snowman didn't think anything of Johnny Carson getting a step ladder and sitting back to front on him," he adds. "On TV, around children, he didn't mind where he was. As for me, I just went along with it. I was just a boy with muddy shoes. I think I've still got them."
De Leyer's own appearance on American soil was unlikely too, making the decision to relocate there with just $160 in his pocket after witnessing a paratrooper being shot down by German troops in front of the brewery that his family ran.
His first foray into work in the US was to work at a tobacco farm before getting the teaching job he kept for most of his working life.
Like the story of Seabiscuit
, Snowman's tale was waiting to be retold to a new generation.
Elizabeth Letts had stumbled across pictures of Snowman jumping and decided to delve a little deeper.
"I came across it by accident really," Letts, a former midwife, recalls of the discovery that ultimately spawned her New York Times bestseller. "It's a remarkable story of a last-minute buy and then the bond of two survivors having both gone through very tough times and a horse that inspired a nation.
"It's a bond that's still there and you can see the tears welling up in Harry's eyes when he still talks about Snowman."
There were others who tried to split that bond, the real estate mogul Bert Firestone telling de Leyer to name his price so he could buy him.
But having doubled his money once before by selling him to his neighbor and immediately regretting it, de Leyer was determined never to let him out of his sight.
"I couldn't sell him," he says. "He'd given me everything. He still gives me everything even in his memory."