Horses Forever: A Sequel to The Horses Know Trilogy in Bloomington, MN

It has always been believed that the people of The Old obliterated themselves generations ago, but when horses begin to amass at one of the city sites of The Old, the villagers of Rockwood discover the truth - an underground city, full of people, has survived.
Supreme City's inhabitants have been waiting for the conditions to be right for them to come up to the surface and claim dominion. They believe the time has come. They are genetically enhanced, armed and aggressive, and they are certain that nothing can stand in their way. But they haven't counted on Will, Maverick, the Horse-Bonded and several hundred horses...