The oldest known horse was called Hyracotherium. It was a small, dog-sized animal that lived in forests and ate leaves. It lived from around 55 million years ago to around 45 million years ago. Horses today are big animals. An adult can weigh from 227 to 900 kg and stand 0.9 to 1.7 meters tall. Horses are grazers that mostly eat grass. Their cheek teeth are tall-crowned, or hypsodont, and are specialized for grazing.

Hyracotherium had 4 toes on each front foot and 3 toes on each hind foot. Over millions of years the ancestors of horses lost toes. Modern horses have only one toe on each foot. A mare gives birth to a baby horse called a foal. A foal can stand up within about an hour of being born. Special tendons and ligaments let a standing horse stay upright without using much muscle effort.

A horse stands on a single toe on each foot, and the toe is covered by a hoof. A horse's hoof wall keeps growing and must be worn or trimmed off, and it grows about 1/4 to 3/8 of an inch a month. A horse can grow a whole new hoof in a year. Horses also have a mane of coarse hair along the neck.

Horses can doze while standing up. Horses are prey animals that depend on running away to stay safe, and a horse uses its ears to hear. Ears pointed stiffly forward mean the horse is alert. Wild horses live in herds of one or two stallions, a group of mares, and their foals. In a herd, some horses lie down while at least one stays standing to watch for danger. People ride horses, and a rider should wear a helmet every time and have a grown-up instructor nearby.

Words to know

• Hyracotherium: oldest known horse
• hypsodont: tall-crowned
• tendons: tough body cords
• ancestors: earlier relatives
• hoof: toe cover