We are all the sum of our genes. When somebody flows with such great and graceful movement as a Jesse Owens, a George Best, a Margot Fonteyn, we search their family tree to see where the inheritance lay. With horses, it is the other way round. Breeders have tinkered for two centuries to try to produce the fastest thoroughbred, but in three-day eventing, which tests the horse and rider over contrasting disciplines on successive days, the tendency is to retrain a hunter, a failed racehorse, a show jumper.

That is changing. The World Equestrian Games start in Aachen in Germany on Wednesday, and there is a British horse called Tamarillo regarded as the most exciting stallion on earth. Part Arab, part Polish, part English, and even with a trace of Hungarian, he was bred in Hampshire and is a unique, pampered, indulged creature. His rider, William Fox-Pitt, has 24 horses in his stable, and not one other is in Tamarillo’s class, or has Tamarillo’s cheeky, self-opinionated, mercurial mentality. “Tam doesn’t think he’s a horse,” says Fox-Pitt. “He thinks he’s human. He’s David Beckham on four legs, obsessed with his appearance . . . ”

The horse’s owner, Mary Guinness, who gave up a Hollywood acting career to marry the Irish writer JP Donleavy, knows and loves the way that Tamarillo plays to the gallery. “He lives and trains with William in Dorset,” she says, “and gets jealous when the other horses have his attention. Deep down, I know he’s much more William’s horse than mine now. It’s like visiting your children, and before a competition, my God it’s stressful.

“I dread the phone ringing. William has learned to start any call by reassuring me Tam is fine because in this sport, the heartache comes in an instant.”

Tamarillo is a product of her devotion and the fascination that Finn Guinness, whom she married after divorcing Dunleavy in 1988, has for genetics.

“My husband,” she says of Finn, whose family founded Ireland’s major brewery, “has a PhD in genetics. He was involved in cloning in Edinburgh, but when he and his sisters were jointly bequeathed Biddesden House it was his opportunity to develop the stud that began here in 1939.

“He’s a horse maniac. He can see a horse once and identify it 20 years later. I’m interested in competition, he in the genetics of the Arab horse.”

Before they married, he suggested she pick any horse in the yard for her own. She chose Mellita, a gentle mare who took her hunting and responded to her merest touch.

Mary grew up in suburban Baltimore and fell in love with Donleavy when she auditioned for a part in one of his plays. She opted for the west of Ireland rather than Hollywood. Their daughter Rebecca and son Rory have flown in from New York to join her when Tamarillo takes on the world.

“You fall in love with horses,” their mother admits. “Tam was born out of my fear of losing Mellita. Before anything happened to the mare, I wanted a foal out of her.”

Mellita was by no means an outstanding athlete, but her bloodline traced back to the pair of Arab horses that established the Biddesden stud near Andover in Hampshire four generations ago. Guinness covered her with Tarnik, the stallion that took his breath away in 1977 when the horse he was riding was beaten into second place by Tarnik at the Arab Horse Society Marathon — 26½ miles across Salisbury Plain. Tarnik was bred at Poland’s state stud in Janow Podlaski. Mary Guinness’s request proved prophetic. Mellita bore two foals by Tarnik: Telemachus, who severed a tendon and was put down when only three; and Tamarillo, who became the last of the line when Mellita was kicked by another horse and shattered a hock.

“He was our orphan,” recalls Mary. “We weaned him, and apart from his handsome chestnut looks, he’s always been so full of life, so sure of himself.”

Maybe he was made that way by his owner’s pampering? “God, yes,” she says.

It would be a whole new question to ponder whether Tamarillo somehow absorbed an actor’s love of the stage. Perhaps the young foal sensed how important, how special he became to Mary Guinness and to his handler, now the stud’s manager, Jo Richardson.

In time, he developed to intermediate events under Diana Burgess, a young rider working nearby with Lucinda Green, before his owner went in search of a male to cope with his boisterousness and draw out his potential. Mary sought out Mark Todd, the outstanding rider of his era. Todd told her he was planning to return to his native New Zealand and since a true partnership between horse and rider at top international level takes five, six, even seven years to establish, he recommended Fox-Pitt.

Mary was doubtful. Her little horse, 16.2 hands high, seemed an odd fit for the 6ft 5in Fox-Pitt who, she points out, was “not the flavour of the month he is today”.

Nobody doubts their chemistry now. They won at Badminton, the biggest and boldest of four-star events, in 2002 — a quarter of a century after Finn Guinness chased Tamarillo’s sire across Salisbury Plain.

At the 2004 Olympic Games in Athens, Tamarillo cantered over the cross-country section as if the course was beneath him. Eventing requires calm obedience on day one, boldness and bravery in the field on day two, and precision to jump in the show ring on the third day, and nothing can be taken for granted.

Fox-Pitt is extraordinarily sensitive to Tamarillo’s soundness. Even as he dismounted after the cross-country, he knew something was wrong. It turned out that the horse had chipped a bone in his stifle, the knee joint in a hind leg. They went from prospective gold medal winner to withdrawal overnight.

At Badminton this year, Tamarillo was withdrawn after a poor dressage test. Outsiders speculated that they were saving the horse for the world games, but both the rider and owner were concerned that the stallion seemed stiff and reluctant on the flying changes in rhythm during the test.

He has since responded to acupuncture treatment to the shoulder and saddle region and is flowing again. “You are going to think me nuts,” says Mary, “but I’ve had a communicator (an equine mind reader) talk to Tamarillo, and I asked my husband to take a cell from Tamarillo in case anything happened to him. People might be against cloning but I wouldn’t be.”

Meantime, in the Biddesden stud, the natural breeding goes on. There is Scorzonera, a five-year-old grey just embarking on eventing, and Sisymbrium, a chestnut two-year-old nephew of Tamarillo who, whisper it in earshot of Mary Guinness, some think might even be better than his uncle.

The Guinness family obviously brew a whole lot more than hops.