There was a prehistoric horse that was the size of a small dog.

          

                Eohippus aka hyracotherium lived between 55 
and 45 million years ago. They averaged 60 cm in length and weighed around 36 pounds. This prehistoric animal has actually been classified in the palaeothere family, which makes it not only an ancestor of modern horses, but also early relatives of rhinoceroses and tapirs.  

           Later on another, now extinct, descendant of hyracotherium called propalaeotherium came about. These animals were even smaller, some only 30 cm (less than 1 foot) in length! These tiny, cat-sized horse-like animals have no modern descendants.