In the wild, horses depend on their hyrarchy for survival. In reality this means that equality does not exist. There is always one horse above the other, the essence of a pecking order.
Horse A(dominant) drives horse B(subordinate to A) of his pile of hay.
Horse B then drives horse C(subordinate to B of his hay pile.
Horse C, in turn, drives horse D(subordinate to C) away from his hay pile.
Finally, horse E drives horse A of his pile!
Or: A>B>C>D>E>A.
To make matters more complicated, when horse B & E are buddies, horse A might have to go elsewhere to make trouble and the hierarchy of relative dominance will shift.
In all of this, much movement is driven naturally daily, hourly, and by the minute, as a result of relative dominance.
(abstract from The Natural Trim ~ J. Jackson)

Relative Dominance is not only real, it is essential