Tracing breed purity into the pre-modern period contends with the fact that the terminology of breed or raza only began to circulate in Spanish vernaculars in the fifteenth century. Re-examining the mid-sixteenth century initiative of the Habsburg King, Philip II, to create a new race of Spanish horses allows for a better understanding of this term in the context of animal husbandry. Raza was a term with new importance in legislation intended to improve the Spanish horse. However, in responses of local breeders collected from throughout the kingdoms of Spain, the term primarily refers to a physical type constructed and maintained through selection and crossbreeding, rather than a statement of purity achieved through inbreeding or breed registry. This analysis demonstrates the fluidity of categories of breed or race, and their relationship to purity, in the early modern period.