The correct answer is - Hyracotherium, Mesohippus, Merychippus, Pliohippus, because over time, the number of toes decreased and body size increased.
The first horses that appeared on the world scene were small plant eaters, living in forests, and having the initial four toes as most mammals had. As the landscape and the climate were changing, the horses had to adapt in order to survive. For the first time in the Earth's history, grasslands appeared, and the forests diminished. The horses were quick to change their diet from leafs to grasses. That gave them more nutrition, so they were becoming bigger, but another reason was that they had to watch out for predators in the tall grasses, so bigger size was an advantage. Also, on the grasslands, in order to escape the predators, the horses had to become fast runners, so the evolution of less toes was in order to have stronger and better support for the legs while running. Eventually that led to the horses becoming hoofed animals, by having only one, hard toe, as we know them nowadays.
