I can't say I've ever seen this particular behavior before. This is not the first…
Social grooming is an important part of a horse’s healthy herd life. It is a way to give comfort and show affection to other herd members. This type of dorsal, neck and wither grooming is said to reduce the heart rate of the recipients, among other benefits.
This behavior is known as reciprocal allogrooming. It occurs in many animal species.
These two colts spend a lot of time together. They have a special companionship.