This is fascinating - I love this kind of history mystery!

"When a Yorkshire archaeology group began excavating some 17th century lime kilns in the Dales National Park, they expected to find remains linked to the once important industry.
Instead, they found a mysterious collection of horse bones buried inside the kilns, which had then been backfilled. Members of the Ingleborough Archaeology Group are now looking for clues to the significance of the strange burial. One theory is that the remains were ritually buried to ward off evil before the kilns were abandoned."
Mystery Of Horse Bones Found In 17th Century Yorkshire Lime Kilns

Burials of horse skulls go back to the Iron Age, at least. Even as late as the 18th century workmen were burying horse skulls in buildings, but the mystery here is why the skulls were deliberately placed when the kilns were closed down.