In the past decades, several scientists endeavored to study and develop biomimetic solutions both for industrial and scientific purposes; nevertheless, only recently, systematic investigations of the bio-inspired design (BID) approach emerged. Even though several BID causal models appeared in engineering design literature during the last decade, none of them has still been adopted as a basis for the development of further design tools and methods. This paper aims at defining a common ontological basis for the functional-causal models by integrating the existing BID models to gain from the synergic effect of their hybridization. The thorough analysis proposed here shows the complementarity of the holistic perspective of the SAPPhIRE representation with the detailed description of system internal structure of the DANE modeling approach; moreover, the ontology of the two models do not conflict with each other and, as such, can be used as a reference to build an integrated model.