The legalization of the creation of human-animal interspecies cytoplasmic embryos, or cybrids, by the Human Embryology and Fertilisation Act of 2008 occasions discourse on significant questions regarding the fixity of species identity and its relevance for moral status. Following a summary of the process by which cybrids are created and the motivation for doing so, I consider Mary Ann Warren’s capacity-based argument as a means of determining moral status and reject it as arbitrary, unduly restrictive, vague, and ultimately inapplicable to cybrids. I then review the biological, phylogenetic, and cohesion species concepts for their potential to define species and address Jason Robert and Francoise Baylis’ contention that even though the lack of a definitive classification system negates the ontological fixity of species, cybrids should not be created because of the moral confusion that they engender by violating a social category with scientific, political, and moral import. I argue that since species distinctions have normative import only if they are ontological and Warren’s capacity-based moral status does not provide an adequate ontological basis by which to differentiate the moral status of humans and animals, the question is one of ontology rather than whether to shore up social categories. The nominalist conviction that species are constructs, attributions of moral status are arbitrary and species distinctions indefensible—which has motivated much of the scholarship on cybrids—must also deny the import of the moral unease about crossing species boundaries and must not be allowed to conflate the epistemic with the ontological definition of species. In response to the category mistake of deducing nominalism from biological evolution I present a brief definition and defense of species essentialism addressing the law of identity, distinction of essence from existence, epistemic access to essence via persistent properties, Aristotelian hylomorphism, and vagueness of species definitions. I contend in conclusion that the moral status of cybrids must be addressed at the level of metaphysics rather than science.