Consider, as an example, the Aberdeen Bestiary, ca. 1200. It is one of the finest surviving examples of the form, and it begins strong, with the lion, the tiger, and the pard (the leopard, it was supposed, was a hybrid of this and the lion). Next up is the panther, which quickly proves to have little in common with the melanistic felines we usually refer to by the term today - an Aberdeen panther is "multicolored, very beautiful, and extremely gentle." It breathes a sweet perfume, which attracts all other animals except the dragon, which cannot bear it and hides in its den. This beast is supposed to have a literal existence, but every feature attributed to it is a metaphor for something about Christ, spelled out explicitly for us by the author.
This exegetic coda is not unique to the panther, by the way - it's found in nearly every entry in the book. The beaver, for example, has only the following description:
There is an animal called the beaver, which is extremely gentle; its testicles are are highly suitable for medicine. Physiologus says of it that, when it knows that a hunter is pursuing it, it bites off its testicles and throws them in the hunter's face and, taking flight, escapes. But if, once again, another hunter is in pursuit, the beaver rears up and displays its sexual organs. When the hunter sees that it lacks testicles, he leaves it alone. Thus every man who heeds God's commandment and wishes to live chastely should cut off all his vices and shameless acts, and cast them from him into the face of the devil. Then the devil, seeing that the man has nothing belonging to him, retires in disorder. That man, however, lives in God and is not taken by the devil, who says: 'I will pursue, I will overtake them...'The more fantastical beasts are given utter credence in the same manner as exotic animals that we now know to exist. The hyena is followed by the bonnacon (a horse-bull hybrid which farts fire), the ape, the satyr, and then the deer - and the deer is given supernatural healing powers when it eats a certain herb, is said to eat snakes. Shortly after we find the unicorn - or, as it is known in this text, the monoceros.
The monoceros is a monster with a horrible bellow, the body of a horse, the feet of an elephant and a tail very like that of a deer. A magnificent, marvellous horn projects from the middle of its forehead, four feet in length, so sharp that whatever it strikes is easily pierced with the blow. No living monoceros has ever come into man's hands, and while it can be killed, it cannot be captured.Note that, unlike later unicorn legendry, it is not purely a graceful horse, and has no particular affinity for virgins.
Other mundane animals follow, and again, to many are attributed unusual features which are then explained as divinely-ordained lessons in Biblical exegesis. The domesticated beasts such as the lamb, horse, and bullock are spared this treatment and described in a straightforward manner, but even something as familiar as the ant is said to divide its stored grain in half, which turns out to be a parable on the nature of Biblical law. The pelican is said to pierce its own breast in times of famine and feed its heart's own blood to its children, so that they will not starve even as the mother bird bleeds to death; this is then explicitly linked to the sacrifice made by Jesus on the Cross.
In these descriptions we see exaggerated travelers tales that have come to be taken as fact (the swordfish, for example, is supposed to so large as to be able to sink ships by piercing them with its snout), legends of faraway lands that are trusted for lack of any better information (such as those of the ape and the bonnacon), and unexpected folklore. But we also see an idea, so basic as to be taken for granted by those living a cloistered life, that all of these things exist for mankind's moral instruction,
We also see a general willingness to accept folklore about animals without observation. Bears are familiar, but few people have seen one give birth, so when the mother bear is said to give birth to a formless cub after a very short gestation and then use her tongue to mold it into shape, it's perfectly reasonable at first blush. (If you saw it in a modern science textbook, you'd probably believe it too - but then, it wouldn't be in a modern science textbook.) No religious explanation is offered. We all know that fisherman who's got a great story about just why it was he couldn't hook that one fish, and huntsmen of the Middle Ages were apparently similar; the deer as presented in the Aberdeen bestiary can eat a particular herb and be healed even from arrows piercing its side, allowing it to run away. This lore is presented as simple knowledge, preserving what was understood as acceptable science in the high medieval period.
Such animal folklore is not lost to us today, though the particulars have changed. The daddy longlegs is not, in fact, a deadly poisonous spider with a mouth too small to actually pierce your skin. Marine biologists don't recommend punching sharks in the nose. Ostriches do not hide their heads in the sand and imagine this keeps them safe from predators. These ideas, however, are a standard part of the folkloric understanding of the animals in today's America, and would have been novel and surprising to the authors of the Aberdeen Bestiary. But they are not able to see our unexpected animal lore, whereas books such as this one allow us to look back and observe theirs, and its curiously religious character.