The behavioral adaptions found in horses include the ability to sleep standing up, the socialization tendencies in herds, the development of long memories and the instinct to flee as part of the fight-or-flight response. Domesticated horses often develop individual behavioral changes based on their environment.
Horses are able to sleep standing up because of the "stay apparatus" ingrained in their leg anatomy. The front legs lock, allowing them to drift off to sleep without falling down. Since horses are prey animals, the ability to sleep on their feet allows them to react quickly to a predator by running away.
Similarly, the fight-or-flight response in horses is to flee. When confronted by a source of anxiety or fear, such as a predator, their first instinct is to run away from it. If flight is not possible, they resort to defensive movements, such as kicking and biting.
Horses' position as prey animals has also made them highly social. In the wild, and even in domesticated situations, they prefer to live in herds. Groups of horses protect one another from predators by mounting retreats and defenses together. Herds also allow horses to protect the weakest members of the group.
Additionally, horses are gifted with long memories. If they experience a negative encounter with an object, human or other animal, they remember it and will demonstrate a fear response when confronted with it again.