The image above shows the evolution of the Horse foot structure from Eohippus wich is 40 million years ago until the Horse in this modern era.
It all starts from the ancient horse called Eohippus that lived 40 million years ago. it has 5 foot fingers, like a monkey foot i think. After 10 million years, Eohippus evolved to a Mesohippus. Mesohippus has a 3 foot fingers wich is different from the Eohippus foot. In the next 15 million years Mesohippus turned or evolved to Merychippus, the last prehistorical horse before evolving to the modern era horse. Merychippus has a 3 short foot fingers, so it looks almost same as the modern horse foot structure. And the last one is the modern era horse that we can see in this world today. It has a flat foot with no foot finger at all.
From the diagram above we can compare the foot structure between ancient horse and the modern era horse, and we can compare the body size between the prehistorical horse and the modern era horse.