There was Dr Jekyll and Mr Hyde. Beauty and the Beast. Victor Frankenstein also had his monster. Without their eternal pairings these individuals were incomplete; with them they achieved a meaningful equilibrium to their existence.
For 12 months racing fans around the world have waited for the reflection of the flawless Frankel to reveal itself at Royal Ascot. His outrageous spread-eagling of a high-quality field hangs in the air, awaiting a comparison.
On Tuesday, it is Animal Kingdom who will line up in the Queen Anne Stakes as the overwhelming favourite to become the yang to the world's greatest thoroughbred's yin.
Frankel's 11-length success in the Group 1 mile contest last year, while Animal Kingdom lay injured for the second time, will be forever his greatest margin of victory at the highest level.
It was his 11th success and he went on to make it 14 without loss when defeating Cirrus Des Aigles in the Champion Stakes at Ascot in October, before retiring to stud.
Animal Kingdom's race record of five victories from 11 starts barely stands up to Frankel's unbeaten sequence. And yet whatever Animal Kingdom achieves in his first competitive run on an undulating and straight mile on turf his performance will inevitably be compared to the English equine colossus.
All tickets have been sold to the Tuesday meet, more out of tradition than to catch a glimpse of the American challenger. Barry Irwin understands that it is futile to try to win over the British crowd. Simply, no horse can fill Frankel's horseshoes.
"I don't want to get into the Frankel thing, because Frankel was on a different planet," the chief executive of part-owners Team Valor International said. "I have perhaps seen only one or two horses that can be mentioned in the same breath as him.
"When you push the button on Frankel he could open up 10 or 15 lengths at a heartbeat. I've never seen anything that could do that at that level.
"Our horse is a very good horse. He is a bona fide horse, but that one is a horse of a lifetime."
If Frankel were a horse of a lifetime, his brilliance has gathered luminosity in the shadow of Sir Henry Cecil's death this week from cancer. After Cirrus Des Aigles had been dispatched it was Cecil who finally sanctioned the media interpretation that Frankel was the greatest horse for the past 40 years. That verdict has not changed in the eight months it took cancer to finally defeat him.
Frankel's privileged background ensured that he had the best possible start. Born and bred on the stud farm of a prince, Khalid Abdullah of Saudi Arabia named him after Bobby Frankel, the legendary American trainer who also died of cancer in 2009.
Frankel was trained by Cecil, who in 2011 was knighted by Queen Elizabeth, and he was managed by Teddy Grimthorpe, a British hereditary peer. There were no such bloodlines to the connections of Animal Kingdom before Shiekh Mohammed bin Rashid, Vice President of the UAE and Ruler of Dubai, became a late addition to the ownership syndicate in March.
Frankel's breeding was both planned and traditional, in that his father, Galileo, the 2001 English Derby winner and now champion sire, was visited by Kind, who had already produced Bullet Train, who would later become his half-brother's pacemaker.
Animal Kingdom's breeding was, in some respects, unintentional and whichever way you look at it, cosmopolitan.
Irwin picks up the story: "I bought the dam for €400,000 in Germany because she beat Soldier Hollow, the 2004 horse of the year, and was gorgeous. When it came time to breed her we had her booked to Kingmambo, but he was having problems with his back so we had to make a decision. We went with Leroidesanimaux" - a Brazilian-bred miler trained in America by Frankel.
Irwin, who was born just three months after Cecil, no longer remembers what he thought of Animal Kingdom when he first saw him, just a few days after March 20, 2008, when the chestnut colt was born a month and a half after Frankel.
In the Team Valor newsletter, Irwin wrote at the time: "This colt has an exquisite head and excellent overall balance."
Where Frankel showed from the start glimpses of his outrageous ability, Animal Kingdom took time to reveal himself. Like a gawky teenager he struggled for looks. He was tall, narrow and thin and lacked the power to win his first race, which was staged six days before Frankel ripped up Ascot with a 10-length success in the Royal Lodge Stakes in September 2010.
Animal Kingdom had a growth spurt during the winter after he had won for the first time, at his second attempt. In Irwin's words the horse "grew like the Incredible Hulk" to land at the 2011 Kentucky Derby with his first competitive run on dirt.
Although Animal Kingdom worked under Robbie Albarado at Churchill Downs four days before the Run for the Roses, he was the first horse to achieve the extraordinary feat of winning the Derby in his first race on dirt.
By denying 18 others he showed also that he could weave through big fields. "I think our derby is an excruciating race, and one of the most difficult races in the world," said Graham Motion, Animal Kingdom's trainer. "It is a very rough race and we threw him into the deep end."
In contrast, the biggest field Frankel ever faced was the 12 runners in the English 2,000 Guineas, staged a week before the Kentucky Derby. Frankel flew out of the gate under Tom Queally and was never challenged.
Where Frankel remained in Britain for the duration of his career and was trained solely from Warren Place in Newmarket, Animal Kingdom's has quickly become the definition of the tough, modern, international racehorse.
After a layoff of nine months Animal Kingdom was unlucky not to push Wise Dan closer than a length and a half in the Breeders' Cup Mile at Santa Anita in November. It was that performance that finally enticed John Messara, the Australian breeder, to buy a 50 per cent share in the horse.
After that run, Animal Kingdom never returned to Fair Hill, Motion's training facility in Maryland where Animal Kingdom was trained on a European-style gallop that rises for the final quarter of a mile.
He was prepared for the Dubai World Cup under the gaze of David Rock, Motion's long-time assistant, in Florida, where his routine consisted of working around flat ovals.
Motion visited Florida perhaps once every two weeks throughout the winter, when Animal Kingdom at his heaviest reached around 550 kilograms.
After losing a prep race at Gulfstream Park in February, Animal Kingdom then became the first American horse to win the Dubai World Cup on Meydan Racecourse's idiosyncratic Tapeta surface.
Motion had chosen Lambourn in England as a base for Royal Ascot because of the quieter environment, in comparison to Newmarket, the headquarters of British racing.
Where Frankel became more relaxed in his old age, Animal Kingdom has become increasingly cantankerous and Motion feared for his charge's mind in Newmarket.
David Nava rode Animal Kingdom in America, but since arriving in Lambourn he has been partnered by three work riders.
Alice Clapham, Motion's travelling assistant, took the reins initially but Animal Kingdom bit off a chunk of her finger that stopped her riding.
Ted Durcan and Peter Carberry have filled in, while John Velazquez partnered Animal Kingdom in his first glimpse of Ascot racecourse last month when he became spooked and jumped the finishing line.
Much like the Mares of Diomedes, it is almost as if Animal Kingdom needs human flesh to calm down as he did not stop at Clapham. Motion has endured a nasty nick to his finger, also.
"He is laid-back but he can be a little tricky," Motion conceded. "He does have a quirky side; he showed that in Dubai. But I think he's been better here." Motion has trained Animal Kingdom up Lambourn's undulating gallops with heart-rate monitors and helmet cameras.
He believes in scales to weigh his horses on a weekly basis and a film crew has followed the horse's movements for the past few weeks.
The horse is a barrel-chested entire that stands at 16.2 hands, large by racehorse standards, and had a screw inserted in his leg after an injury to his hock. The screw remains there.
If you look carefully at his only white sock, on his rear left leg, there is a scar on his fetlock joint where the veterinary surgeon made his way in.
Frankel, has no such scars, mentally or physically, and is more like a bay panther, low-slung with a raking stride. How much Frankel weighed as a racehorse is academic because Cecil never weighed his horses, never knew how fast their hearts were pumping and never considered watching a gallop from the jockey's perspective.
In all probability, the Queen Anne Stakes is Animal Kingdom's final race.
Although Messara has tantalisingly left the door ajar to the remote possibility of another race, Animal Kingdom's fee at stud has been set. For $A38,500 (Dh135,700) you can have your mare endure a tryst with Animal Kingdom, either in Australia or at Darley's facility in Kentucky, as he is to shuttle between hemispheres. Frankel, on the other hand, is already at stud, with Juddmonte charging him out at an eye-watering £125,000 (Dh719,000) and, much like during his racing career, he is lodged within the confines of Newmarket.
On Tuesday we will see just how good Animal Kingdom is as he bids to become the first Kentucky Derby winner to compete at the Royal meeting since Omaha finished second in the Gold Cup 77 years ago. If he wins his first mile event to register his third Group 1 success on a third continent on a third different surface he will have achieved greatness via a route Frankel could never have trod.
Where Frankel was perfect, Animal Kingdom is scarred, where Frankel's record is flawless, Animal Kingdom has been beaten. But Frankel was essentially a noble British project, whereas Animal Kingdom's owners are spread across the planet around which he has been campaigned.
As Shakespeare wrote in As You Like It, "Some are born great, some achieve greatness, and some have greatness thrust upon them."
The capacity crowd at Ascot on Tuesday will have had something very special thrust upon on them should Animal Kingdom prevail.