Aristotle's metaphysical theory of individual objects and their essences is preeminently aimed at formulating and resolving several fundamental issues regarding biological objects, living things. For this reason the biological writings are in some ways a better resource than the Metaphysics for a vivid intuitive conceptualization of what 'substantial being' concretely comes to.
Furth, Montgomery, "Morphogenesis: Some Elements of Aristotelian Embryology and Some Ontological Ramifications" (1975). The Society for Ancient Greek Philosophy Newsletter. 92.