Reports of women giving birth to a baby together with an animal (toad, mouse, bird, etc.), are documented in Europe from the 1100s onwards: the most important traditions of which are the frater Salernitanorum and the sooterkin. Throughout the centuries, authors have typically attempted to explain monstrous animal siblings in the light of contemporary medical knowledge. The present paper compares the medieval frater Salernitanorum with the later sooterkin and investigates both in historico-folklore terms. It argues that it is important to understand monstrous birth traditions not only in the light of medical history, but as beliefs and narratives actively shared through acts of communication. In so doing, this article is informed by both the history of medicine and folklore studies.