"I was out riding one day and I spotted a baby lamb feeding," equine veterinarian Georgina Hirst told Britain's Sunday Telegraph. "The first time I saw it I couldn't quite believe it -- I thought I might be imagining it. But then I saw it again and again over the space of about a week."
More than 2,000 sheep and 60 horses roam the Black Mountains in Wales, the Telegraph said, but the Shetlands are not normally so accommodating.
"Trying to get mares to adopt foals can be very challenging so it's incredible the mare was so receptive of the lamb," Hirst said. "She would even stand guard while the foal and the lamb slept cuddled together."
Hirst returned the lamb to its farmer when she realized that both the lamb and the foal were not getting enough food from the pony.
"The mare simply wasn't producing enough milk to feed the both of them," Hirst said.