The aim of this paper is to contribute to the body of animal studies scholarship by attending to a lesser-known area of medieval literature: Middle Welsh. The earliest collection of prose literature in Middle Welsh, titled The Mabinogion, is both prolific and complex in its depiction of animals, making it a fruitful site for analysis. After reviewing the context of The Mabinogion and the intellectual and cultural milieu which may have informed its writing, I consider how the text’s portrayal of the non-human reflects, refutes, or expands these Christian and Celtic influences. In analyzing scenes in the Four Branches and the tale How Culhwch Won Olwen which involve human-animal encounters and transformations, I suggest that The Mabinogion combines these strains of thought into both expansive and definitive understandings of the non-human animal.