This dissertation identifies the presence within early medieval Britain and Ireland of a specific discourse around animality, which is labeled ‘the physiologic tradition’. Derived partially from the Late Antique Physiologus, a collection of religious animal lore, the physiologic tradition fundamentally combines various forms of ontological alterity—animality, wonder, and ethnicity being three key manifestations of that alterity—with instruction in correct spiritual and doctrinal beliefs. Examining a variety of early medieval texts in which physiologic animals are found, including the Old English Phoenix, the Irish Latin Navigatio sancti brendani (‘Voyage of St Brendan’), the Exeter Book riddles, and Gerald of Wales’ Topographica Hibernica (‘The History and Topography of Ireland’), this dissertation tracks how various Christian authors used these literary animals to explore questions of theology, ontology, and the natural world, as well as to dehumanize and harm ethnic and religious others.