Stallions.
The foundation of every herd, you say?
No, unfortunately. But they are the glue.
The top ranking stallion is Equinox: he wields the most power and holds the most mares in his command. Each stallion sires approximately two-five foals before he dies and becomes too old to carry on. Every herd has a lead mare and lead stallion; the stallion protects them and sires all the herd's offspring, while the mare does most of the commanding and decision making.