Claudius Aelianus (ca. 175 – c. 235 CE), commonly known as Aelian, was a Roman author and teacher of rhetoric. He is the author of On the Nature of Animals (or On the Characteristics of Animals, or in Latin De Natura Animalium), a work in 17 books. In it Aelian describes many animals, both real and imaginary, not usually distinguishing between the two. Much like Pliny the Elder in his Historia naturalis, Aelian uses an array of ancient Greek and Roman authors as his source, along with traveler's tales and myths.

"Aelian's anecdotes on animals rarely depend on direct observation: they are almost entirely taken from written sources, often Pliny the Elder, but also other authors and works now lost, to whom he is thus a valuable witness. He is more attentive to marine life than might be expected, though, and this seems to reflect first-hand personal interest; he often quotes "fishermen". At times he strikes the modern reader as thoroughly credulous, but at others he specifically states that he is merely reporting what is told by others, and even that he does not believe them. Aelian's work is one of the sources of medieval natural history and of the bestiaries of the Middle Ages." - [Wikipedia]

There is no structure or organization to Aelian's work. Animals are often described in multiple chapters, as though Aelian simply wrote down (continuing from wherever he last stopped) the latest thing he heard about an animal or whatever was suggested by the tale of some other animal. In some cases a later description of an animal differs from earlier descriptions. There is no order to the chapters. On the Nature of Animals is in no way an encyclopedia of facts about animals; it is more like a notebook where the author wrote down stories as he heard them.

Aelian defends his chaotic accounts at the end of Book 17:

I am aware too that some will express disapproval because I have not in my discourse kept each creature separate by itself, and have not said in its own place all that is to be said about each, but have mixed the various kinds like a varied pattern in the course of describing a great number, at one point dropping the narrative about such-and-such animals, at another going back and stringing together other facts about their nature. Now in the first place, speaking for myself, I am no slave to another's judgment and will: I maintain that it is not my duty to follow another's lead wherever it may take me. And in the second place, since I was aiming to attract through the variety of my reading matter, and since I flee from the tedium arising from monotony, I felt that I ought to weave the tissue of this narrative of mine so as to resemble a meadow or a chaplet beautiful with its many colors, the many creatures, as it were, contributing their flowers. And although hunters regard the finding of even one animal as a piece of luck, I maintain that there is nothing splendid in finding the tracks or capturing the bodies of such a multitude of animals, whereas to track down the faculties which nature has seen fit to bestow upon them - that is splendid. - [Scholfield translation]