Cat-sized horses roamed the world 50 million years ago!

If one were to go back 50 million years in time, they'd find the Earth to be much hotter than it is today. One would also discover that horses, the size of house cats, roamed the forests of North America. Bronies would've been so happy. 

These earlier species of horses, known as Sifrhippus, grew smaller over many thousands of years in order to adapt to the rising temperature of a time when methane emissions were extremely high, possibly due to major volcanic eruptions.