A horse can be brought along and kind of trained to be a future leader by the herd mates that they are with and by the experiences that the horse has but true leaders are born.
Most good leader are a bit aloof and are a little more secure in themselves.
They have more confidence and have the ability to past that on to the other members.
If a horse has a hard time getting along and being a good herd member then they live alone for a while until they learn to appreciate the safety and comfort of the group. This sometimes helps but is no guarantee.
Some of the horses that can not be combined are "Made" by the way they have been housed and the horses that they have been with.
They were more than likely beat up a lot as young horses and now it is pay back time.
Insecure horse are easier to work with in some ways as they are always waiting for the hammer to fall and are very space conscious.
They can also have a lot more baggage such as weaving,sucking air,timid on the trail, and very nervous during learning.
So people feel more secure around insecure horses as they feel more in control.
So I think many of these horses are created by their environments and the owner may or may not even be aware that it is happening.
The dynamics are endless and every herd is different.
"The greatest strength is gentleness."
- Iroquois Proverb