The diagram illustrates the evolution of the horse, occurring over a geologic time scale of 40 million years, transforming small-sized Eohippus into the modern horse.
Overall, the lineage leading from Eohippus to the modern-day horse exhibits some evolutionary trends, including increase in size, reduction in the number of hooves, lengthening of the legs, and elongation of the muzzle.
About 40 million years ago, with a relatively short head and neck, Eohippus developed five toes, of which four were equipped with small hooves, while the fifth was off the ground. Living about 30 million years ago, as an intermediate between the Eohippus-like horses and more “modern” horses, Mesohippus was slightly larger and leggier, and walked on three toes.
By 15 million years ago, Mesohippus had evolved into a somewhat larger form known as Merychippus, representing a milestone in the evolution of horses. Though it retained the primitive character of 3 toes, it looked like a modern horse, with a long face and a newly-developed mane. Finally, the modern-day horse evolving from Merychippus now has a single hoof, slender legs, along head, a relatively long neck, and a long tail.