Calm Healthy HorsesHerd-Bound / Separation Anxiety 
It is normal for horses, because they are herd animals, to have a natural inclination to want to go with the group.
It is not normal for them to be ridiculous about it!
It is not normal to tear around ripping up the paddock getting themselves in a complete lather, even jumping fences in a desperate effort not to be left behind when their paddock mate is taken out.
It is not normal for them to completely lose the plot on a trail ride when their friend is only five yards away.
Abigail's story is typical of many herd bound horses and proves it is not about a horse not being able to be left alone but one of a chemistry imbalance - easily fixed with the correct diet.
Overview
In fact this is the worst thing you could do because it is so mentally cruel on the poor horse.
Horses are not meant to be on their own. You wouldn’t do that to an insecure child would you?
I have even seen weanlings taken straight off their mother and left alone in a paddock to teach them this lesson. This practice has actually been proven to have the exact opposite effect and cause the very insecurity you are trying to avoid.
What to do?
Irrational, over the top herd-bound behaviour is caused by mineral imbalances affecting the central nervous system of the horse to the point they can’t think straight.
Sometimes it’s like they can’t think at all! So they can’t learn whilst in this state either.
Fix the diet so the horse can function normally and then whilst your horse might still express the desire to go with his friends, you can easily ‘talk him out of it’.
This behaviour diminishes and eventually goes away completely with the diet change.
Mystery’s Story is a good example of a herd-bound horse coming right with the change of diet!
Company
Horses must have the company of other horses! They are tactile animals who enjoy being able to touch and groom each other as well as gaining a sense of safety from the group.