The evolution of the horse started with Eohippus during the Eocene period. It involved the following evolutionary stages.

$\Rightarrow$ A gradual increase in body size
$\Rightarrow$ Elongation of head and neck region
$\Rightarrow$ Increase in the length of limbs and feet
$\Rightarrow$ Gradual reduction of lateral digits
$\Rightarrow$ Enlargement of the third functional toe
$\Rightarrow$ Strengthening of the back
$\Rightarrow$ Development of brain and sensory organs
$\Rightarrow$ Increase in the complexity of teeth for feeding on grass

The evolution of the horse is represented as

(i) Eohippus
It had a short head and neck. It had four functional toes and a splint of 1 and 5 on each hind limb and a splint of 1 and 3 in each forelimb. The molars were short crowned that were adapted for grinding the plant diet.

(ii) Mesohippus
It was slightly taller than Eohippus. It had three toes in each foot.

(iii) Merychippus
It had a size of approximately 100 cm. Although it still had three toes in each foot, it could run on one toe. The side toe did not touch the ground. The molars were adapted for chewing the grass.

(iv) Pliohippus
It resembled the modern horse and was around 108 cm tall. It had a single functional toe with a splint of 2nd and 4th in each limb.

(v) Equus
Pliohippus gave rise to Equus or the modern horse with one toe in each foot. They have incisors for cutting grass and molars for grinding food.