These earliest known horses, known as Sifrhippus, actually grew smaller over tens of thousands of years in order to adapt to the higher temperatures of a period when methane emissions spiked, possibly due to major volcanic eruptions.
The research could have implications for how the planet's modern animals may adapt to a warming planet due to climate change and higher carbon emissions, scientists said.
Researchers made the discovery after analyzing horse tooth fossils uncovered in the western US state of Wyoming that showed the older ones were larger, and that the species had shrunk over time.
Many animals became extinct during this 175,000-year period, known as the Paleocene-Eocene Thermal Maximum, some 56 million years ago.
Others got smaller in order to survive with limited resources.
"Because it's over a long enough time, you can argue very strongly that what you're looking at is natural selection and evolution -- that it's actually corresponding to the shift in temperature and driving the evolution of these horses," said co-author Jonathan Bloch of the Florida Museum of Natural History.
Average global temperatures rose by about 10 degrees Fahrenheit during that span due to massive increase in carbon that was unleashed into the air and oceans.
Surface sea temperature in the Arctic was about 23 Celsius (73 Fahrenheit), much like the temperatures of contemporary subtropical waters today.
The research showed that Sifrhippus shrank by almost one third, reaching the size of a small house cat (about 8.5 pounds, four kilograms) in the first 130,000 years of the period.
Then, the horses grew larger again, to about 15 pounds (seven kilograms) in the final 45,000 years of the period.
About a third of known mammals also minimized themselves during this time, some by as much as one half.
"This has implications, potentially, for what we might expect to see over the next century or two, at least with some of the climate models that are predicting that we will see warming of as much as four degrees Centigrade (seven degrees Fahrenheit) over the next 100 years," said co-author Ross Secord of the University of Nebraska-Lincoln.
Some birds have already been observed to have become smaller in size than in cooler times in the past, he said.
However, the forecast changes are expected to happen over the next century or two due to a boost in carbon emissions dating back to the start of the Industrial Revolution.
Millions of years ago, the climate change happened much more slowly, taking 10,000 to 20,000 years to get 10 degrees hotter, he added.
"So there's a big difference in scale and one of the questions is, 'Are we going to see the same kind of response?' Are animals going to be able to keep up and readjust their body sizes over the next couple of centuries?"