Folk biological classifications and the taxonomic schemes of scientific biology have often been conceived as two monoliths that sometimes correspond and sometimes do not—as, for example, when folk zoologists classify whales as fish. The Lio people of Flores Island describe dugongs (Dugong dugon) as creatures that are half human and half fish, thus, essentially like the European image of mermaids. The characterization relates to a myth, widespread in Southeast Asia, which depicts the animals as deriving from a woman. At the same time, Lio speak of dugongs as, simply, a kind of fish. This apparent inconsistency is reflected in several ways people name dugongs, as well as in sex-differentiable terms and numeral classifiers employed when speaking about the animals. Reviewing different ways Lio describe dugong morphology, this nomenclatural variety is shown to correspond to three complementary models identified as diametric, concentric, and chronological dualism. Finally, I demonstrate how these models are comparable to competing ways of representing relationships among animals in modern biological systematics and discuss the implications of such parallels for ongoing debates about similarity and difference between folk and international biology.