Six-year-old Koko is one of a family of three with a rare genetic mutation that makes them intersex horses. (Canadian Press)

Sam Campbell was baffled when her mare Koko began acting in a decidedly unmare-like manner, aggressive to people and trying to mount other females in the herd like a stallion. It turns out that Koko was no lady in more ways than one.

Scientists at the University of Guelph discovered that the six-year-old animal is a rare intersex, or sex-reversed, horse.

Outwardly, Koko has all the characteristics of a mare but was born with internal testes that were juicing her system with testosterone and making her act like a stallion.

"I'd been training her and she'd be fine and the next day she'd go wild. Her behaviour was very erratic," said Campbell, who owns four other horses at her hobby farm near Cobourg, Ont., east of Toronto. "Then she started showing stallion-like behaviour, urinating on other horses' feces, mounting the mare horses."

"I'm thinking, what is wrong with this horse?"

Campbell said a local veterinarian she called in performed an ultrasound examination on Koko and found what looked like internal testes, a fact confirmed when the horse was taken to the University of Guelph's Ontario Veterinary College.

'What's wrong with this horse?'

Tracey Chenier, an equine reproduction expert at Guelph, said stallion-like behaviour in a mare is more often the result of an ovarian tumour, so the vets examining Koko were somewhat surprised to find the horse was actually an intersex animal.

It was no wonder Koko became a problem when she was turned out with a herd of mares, said Chenier, "because she was a he, and rightly so acted like it."

But Koko's anatomy wasn't the only eye-opener for the Guelph specialists working on the case — they found out Koko was not the only one.

"I was very surprised when Sam said, 'I have another one at home that I think is the same,"' said Chenier, explaining that Koko's younger sister Sequoia had been born to the same mother, Angel, which Campbell had purchased at a horse sale in nearby Lindsay, Ont.

An examination of Sequoia showed the filly also had internal testes, which like Koko's were surgically removed. They then discovered that Angel's sister, who had been bought by a different owner, had a female foal named Pandora that was also an intersex horse.

Allan King, a specialist in animal reproductive biotechnology at the University of Guelph, said it is extremely rare to find three animals with the same genetic disorder.

"That's why we're quite interested, because it's a family rather than a random occurrence," he said. "We believe that it's inherited on the X chromosome. So their fathers were normal male stallions and their mothers are carrying this gene, which they're passing on to their offspring."

"It only comes to light when one of these animals is examined in a clinic."

Extremely rare

King said Koko, Sequoia and Pandora cannot produce foals, as they have no cervix or ovaries. But their mothers, which likely carry a mutated gene on one of their X chromosomes, could theoretically pass it on to future offspring. The female foals would either become carriers themselves or be born as sterile, sex-reversed animals.

"In the animal world, we really have no way of following up on this," he said. "We know, for example, in this family the two females [Angel and her sister] must be carriers to pass it on to their offspring, but we don't know whether there are others out there in the family that are carrying the gene or not."

As for Koko, her owner says the horse will always be a mare to her.

"In my eyes, she was born a girl, she had some boy parts, but on the outside she looks like a girl," said Campbell, adding that since the removal of the testes, Koko is like a different horse.

"She's amazing ... Now she's very docile, she's a lovely horse, so much so that a friend's daughter asked if she could work with her and take her into the local schooling show," she said. "It's been a wild ride."