The aim of this paper is to trace the development and relationship between Polish wydra and English otter in a broader Indo-European context. The methodology of the research involves three steps: gathering cognates (to determine the time and place of attestation), identifying morphological structure and describing the sound changes that have occurred in two descending lines of development: one, from ProtoIndo-European *ud-r-eh2 leading to Polish wydra, and the other, from Proto-Indo-European *ud-r-o- to English otter. The analysis leads to the conclusion that the word for ‘otter’ in Proto-Indo-European must have had distinct masculine and feminine forms and, structurally, represents a substantivized adjective meaning ‘aquatic’: its root was the zero-grade form of PIE *uod-r/n- ‘water’ and the -r- suffix used to perform the adjectival function.