Isidore died before finishing his Etymologies, divided into twenty books by his friend Braulio, bishop of Saragossa. The title reflects his basic literary interests, and his method often leads him into fanciful word-derivations which he considers scientific. He discusses animals at the end of Book XI and in the whole of Book XII, and is less credulous than the author of the Physiologus. He has avoided many legendary anecdotes because he has analyzed narratives in the manner of Greek rhetoric, dividing them into three classes defined as historical fact, fiction, and myth.
For Isidore historical “facts” really took place, and even if “argumenta” (fictitious accounts) did not occur they could have occurred. Fables (myths) did not occur and cannot occur, however, because they are contrary to nature. The Physiologus, of course, had paid no attention to such distinctions, but Isidore was better trained in rhetorical analysis and more concerned with it. Though he discussed many of the fabled creatures found in the Physiologus, he did not often classify them as “animals.” Relying on Varro (through Augustine), he placed “monsters” and “fabulous portents” at the end of the eleventh book (or did his editor Braulio do this?), accepting the first group of portents as trustworthy (11.3.1–27), and even (like Pliny) citing Aristotle as an authority. These stories are placed under the heading “portents” and are different from the materials “on animals,” but they are also different from a few fabulous and fictitious accounts which can be explained away (11.3.28–39). Isidore definitely believes that transformations of men into beasts, or vice versa, are possible, and it seems surprising that he accepts the existence of vampires (11.4).
Henkel notes Isidore’s criticism, possibly after Augustine, of the tales about the weasel and the pelican and his references to the existence of hearsay. Isidore’s work is somewhat more “scientific” than the Physiologus, and Henkel rightly insists that medieval people did not regard the latter as a textbook of zoology. It is not what we should call scientific, however, for it is based on neither observation nor analysis but simply on rhetorical tradition.

Robert M. Grant. ‘Early Christians and Animals’, pp. 113-114. Routledge, 1999.