Historians need to understand the nature of historical agency and how animals relate to this central if contested historiographical concern. Focusing on the specific context of the Napoleonic Wars and in particular the Duke of Wellington and his horse Copenhagen, I show why agency is a continuum, not limited to the complex and intentional acts of a rational man, for instance a field marshal, but extending to basic actions, group actions, and less self-conscious actors, for instance a horse. Therefore, agency can include animals. Any action, however, must be placed into a context of reasons or understandings, the “pertinent context.” The place of animals as agents will naturally vary across historical time in great part depending on prevalent contemporary cultural assumptions. In some periods animals have operated as difficult-to-discern “secret agents.” I stress the variable and fluid notions of agency that have emerged in posthumanistic and actor network theoretical contexts. And I develop the idea of special associations—what I call unities—in which especially close, disciplined actors are produced, such as the skilled horse-and-rider of the nineteenth century. Ultimately, historical agency is likely always to involve human beings, but there is also space for animals to act with people.