Horses are Well Adapted to a Herd Lifestyle
The structure of the herd provides the horse with his basic security — safety in numbers. The horse is a flight animal and in the face of danger, will take fright and ran away. If one member of the herd senses danger and takes off, the rest will follow.
Even domestic horses out in the field maintain their basic herd instincts. Horses generally stand and graze fairly close together, and if one horse is apart at a distance, it is often an indication that something is wrong and should be investigated.
Horses are well adapted to a herd lifestyle and communicate with each other using body language, such as ear movements and vocal sounds. They sleep standing up, which allows them to retreat quickly in the face of danger.
In cases where horses are seen lying down, there will often be one or two standing up on 'guard duty,' ready to alert the rest of the herd if need be.