The pictures show how the horse has evolved over the course of 40 million years.
Overall, the size of the horse has increased significantly with the emergence of new features such as manes and hooves.
Forty million years ago, the Eohippus, which was the first in its lineage, had a short neck and tail, and it had four similar-sized toes on each foot. Ten million years later, the Eohippus had evolved into the Mesohippus, which was almost double its ancestors in terms of size, tail, and neck length. The foot structure of the later creature had also changed as it had only 3 toes, with the middle one being bigger than the side ones.
About 15 million years ago, the Merychippus, which had evolved from the Mesohippus, had a discernible mane and the tail was a bit thicker and longer than that of the Mesohippus. Its body size and other features were similar to those of its closet ancestor, but the toes of the Merychippus had become much bigger, with the size of the middle one doubling that of the other toes. Finally, the present-day horse had become a bit taller and larger than the Merychippus. It had lost its side toes and developed a large hoof. Its tail and mane are distinctively longer than those of its ancestors.