Unicorns may be magical, majestic, beautiful, etc., but they have a serious weakness: virgin women. Indeed, ancient lore says that using a maiden as bait is the only way to snag yourself one of these elusive beasts. When the unicorn falls asleep in her lap, you can capture it, or if you're feeling particularly plucky, run it through with a spear.

That was far from mere fantasy back in the Middle Ages, though. The story was one of the more famous passages in what are known as bestiaries, gorgeous compendiums of creatures both real and imagined that sold like mad—second only to the Bible itself. While they were passed off as solid knowledge, bestiaries were almost always wildly wrong about the natural world. Nonetheless, these charming tomes were indispensable to the beginnings of modern science, helping lay the groundwork for the field of zoology as we know it.

Unicorn murder notwithstanding, bestiaries tended to hold a certain reverence for the natural world, ascribing the cleverness, not to mention vices, of humans to various animals. The beaver, for instance, was said to chew its own testicles off when pursued by hunters. And the asp could resist the snake charmer’s song by putting one ear against the ground and plugging the other with its tail.

Adorable little hedgehogs roll around on fruit to stick them to their quills. They'll then store them in their den for the lean times. Rochester Bestiary/British Library. Used with permission.

Each creature usually was explained in just a few paragraphs, followed by an often lengthy moral tale to relate its biology or behavior to human beings. So in casting off its testicles and throwing them in the hunter’s face, the beaver thus relieves himself of vice and tosses it right back at the devil. By plugging its ears, the asp is unwisely resisting the word of Christ.

A few more examples of fantastical bestiary critters, which you can see in the gallery at top:

• To steal a tiger cub, a hunter must grab a bunch at once and make off on a swift horse. As their furious mother closes in on him, he drops a single cub, which the mother snatches up and takes back to her den. She thus returns to the hunter for one cub at a time, until he escapes on a ship, ideally with at least one left.

• The magical salamander was said to be fireproof as well as extremely poisonous, tainting wells and even fruit on trees. Indeed, when a salamander got into a river that Alexander the Great’s army drank from, 4,000 men and 2,000 horses supposedly grew sick and, no doubt weakened by the embarrassment of being bested by a salamander, keeled over.

• Crocodiles may be jerks, but they’re far from invincible. As they sun themselves on the shore, mouths agape, the serpent will crawl in and make its way into the crocodile’s guts, eventually bursting through the poor reptile.

For all of the popularity of bestiaries in the Middle Ages, strangely enough one wasn’t translated into English (most were written in Latin) until 1954: T. H. White’s seminal Book of Beasts. In it, White goes to lengths to emphasize that while bestiaries were wildly wrong nearly all of the time, they were in fact quite compassionate works, painting animals as creatures to be not only respected, but revered.

The fox was said to roll itself in red mud to appear blood-stained, then flop over on its back and hold its breath. The scavenging birds that come to investigate it are thus bamboozled (read: devoured). Rochester Bestiary/British Library. Used with permission.

Now, it’d be an understatement to say there was a bit of an illiteracy problem in the Middle Ages. Consequently, bestiaries featured incredibly vivid, lively illustrations that spelled out the behaviors of the various creatures. Above, for instance, is a fox, a “fraudulent and ingenious animal” that was said to play dead to attract and attack birds. A medieval illiterate peasant would have heard such a cautionary tale in church, for the devil metaphorically works in much the same way on humans. Thus was the bestiary a work of universal accessibility.

Such symbolism was so important in the Middle Ages, according to White, “that it did not matter whether certain animals existed”—the part man, part lion, part scorpion with probably some identity issues known as the manticore, for instance—but “what did matter was what they meant.” It was an era of intense faith that a higher power had created every creature with a meaning to be decoded by man, or in the case of plants, a clue that the species was meant to treat a certain organ. Known as the doctrine of signatures, this held that a walnut, for instance, could treat brain problems because it looks an awful lot like a brain itself.

The bestiaries can all trace their lineage to one masterwork: the Physiologus, whose anonymous author probably lived in Alexandria in the first few centuries AD. He or she dug deep into the animal lore of the ancients, combining thoughts of Aristotle, who was arguably the first true scientist (if you haven’t yet picked up Armand Marie Leroi’s fantastic new book The Lagoon: How Aristotle Invented Science, do so immediately), as well as Pliny the Elder, who in compiling an encyclopedia of all Roman knowledge was sure to include epically fantastical animal "facts," I guess just in case they turned out to be true.

Not only is the asp exceedingly deadly to humans, it’s far from easy to charm. When snake charmers try to lure it out of its hiding place, the asp will press one ear against the ground and plug the other with its tail. White’s bestiary translation notes: “Apart from men, asps are the only other creatures which do such a thing, namely, refuse to listen.” Worksop Bestiary/British Library. Used with permission.

The Physiologus was a huge hit. It was translated into everything from Syriac to Latin to Icelandic, spreading all over Europe and the Middle East and Africa. And when a scribe got his hands on it, according to White there was no reason he “should not expand his copy with interpolations” from later authorities. Thus the bestiary “began to grow like a living tree.” Later authors grabbed older bestiaries and tacked on extra bits. They cut things here and there. In the words of White, the bestiary was “a kind of naturalist’s scrapbook, which has grown with the additions of several hands.”

This went on throughout the Middle Ages until the 16th century rolled around, and naturalists started growing suspicious of the bestiaries' claims. Then came along British polymath Sir Thomas Browne, science's most accomplished party pooper, whose work Vulgar Errors ripped the bestiary’s many bizarre claims to pieces. Elephants do indeed have knees, he assured his readers, and beavers aren’t anywhere close to being capable of chewing off their own testicles. And in busting these myths, according to White, Browne “began to raise the subject of biology to a scientific level for the first time since Aristotle.”

Generations of natural historians followed Browne, trusting not in the strange moral tales of their forebears, but in direct observation. And in 1859, the science of biology tallied what is arguably its ultimate triumph with Darwin’s publication of On the Origin of Species. Far from the fanciful speculations of the bestiaries, science could now explain not only the deep biology of animals, but how and why they got that way.

So today we find ourselves owing an unlikely debt to some of history's most erroneous works. Plus, the next time you come across a unicorn, you'll know exactly what to do.

Check yourself into rehab immediately.

Browse the full Fantastically Wrong archive here. Have a crazy theory or myth you want me to cover? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.

References:

Badke, D. (2010) The Medieval Bestiary

Curley, M. (1979) Physiologus: A Medieval Book of Nature Lore. University of Chicago Press

Pliny the Elder (1855) Natural History. Taylor and Francis

White, T. H. (1960) The Bestiary: A Book of Beasts. Capricorn Books