Granted, they may both fall under the generic heading of “animal,” yet that universality exists only as a mental abstraction, not as something instantiated outside the mind.
To clarify: the distinctive marks of animality are sense perception and voluntary motion. A horse’s powers of perception are not like those of a human being—indeed, they are not even like those of a cat or a mouse, despite their common animality. Nor is its voluntary motion identical to the voluntary motion of those other creatures. Had a human truly matched every other animal in perception and voluntary motion, each would necessarily possess all the concomitants entailed by the other’s faculties; for it is impossible for a substratum to exist while lacking its necessary concomitant. Yet the perception and voluntary motion of a human being entail concomitants that a cat or a mouse could never possess, and vice-versa. We thus know that their perception and motion differ in kind, just as their underlying realities differ in kind; therefore their animality differs in kind as well, and things that differ in kind and in essence cannot be identical. Their common participation in the genus “animal” is no different from black and white both belonging to the genus “color,” even though they differ utterly in kind and in reality.
Moreover, every attribute of perfection that belongs to the Lord, exalted be He, is inseparable from His very Essence. Were another essence to resemble His, it would have to share everything with which the Lord is characterized: it would be All-Knowing, All-Powerful, capable of creating a world like this one, and so on—consequences whose impossibility is evident.
Again, if the Lord—transcendent is He—had an equal, that equal would have to be able to do whatever He can do and will whatever He wills. In such a case the existence of the world would be impossible. For if it were possible for each of the two to act independently, it would follow that each would be the sole agent of the whole, while in fact neither would bring anything into being.