Feral Empire explores how horses shaped society and politics during the first century of Spanish conquest and colonization. It defines a culture of the horse in medieval and early modern Spain that, when introduced to the New World, left its imprint in colonial hierarchies and power structures. Horse populations, growing rapidly through intentional and uncontrolled breeding, served as engines of both social exclusion and mobility across the Iberian World. This growth undermined colonial ideals of domestication, purity, and breed in Spain’s expanding empire. Drawing on extensive research across Latin America and Spain, Renton offers an intimate look at animals and their role in the formation of empires. Iberian colonialism in the Americas cannot be explained without understanding human–-equine relationships and the centrality of colonialism to human–-equine relationships in the early modern world.