My dissertation examines horse breeding in the early modern Spanish empire (1490s-1580s) and its contributions to historical processes of racialization in Spain and Latin America. In my research, I ask how the practices of horse breeding contributed to the vocabulary of race (raza) and caste (casta), considering the shared concepts of generation and reproduction that were used to explain hereditary features among human and animal populations, and the role of the horse in symbolizing and embodying social distinctions in the early modern period. I propose that what I call the "experiential practice" of horse-breeding tested definitions of race and caste against the physical constraints of environment and physiological features of animal reproduction. In Spain, regulation of horse reproduction for the 'common good' in municipal and royal legislation combined horse breeding practices with pre-existing discourses of nobility in terms of caste and race, and by the late sixteenth century, King Philip II would create a new program to breed a "pure race" horse. In the Americas, a discourse of fertility and unregulated horse reproduction served as a stark contrast to the discourse of scarcity and the promotion of horse breeding in Spain, questioning the relative influence of environmental and hereditary factors on animal generation and serving in debates over purity, incest and miscegenation. Historical studies of racialization have previously demonstrated that a transfer of racial terminology from horses to people formed part of the ideological and discursive power of race, without investigating historical frameworks of "race" among animals. Thus, horse-breeding serves as a innovative lens for understanding the use and development of racial terminology among animal and human populations in early modern Spain and in Spanish colonization in the Americas, not as an imposition from one on the other, but as sites of knowledge making where practice and discourse interact.