In ancient times, humans had some pretty strange theories about animals and none of the ethics that would stop us from testing them out.
Pliny the Elder, for example, wrote the encyclopedic Naturalis Historia (Natural History), which collected together most of the knowledge of animals and plants from the time (up to AD 79 when he died in the eruption of Mount Vesuvius). Among the established “truths", he claimed that entire beehives die upon contact with menstrual blood and that elephants and rhinos are mortal enemies.
This latter belief lasted for centuries and was eventually put to the test by Dom Manuel I, the king of Portugal in the 16th Century. He was so convinced that Pliny was correct that he arranged to have a rhinoceros fight one of his elephants. The fight was ruined when the elephant merely ran off.
Some of the weirdest beliefs about animals were on display for all to see in the Tower of London. During the reign of King John, the English began (very, very slowly) to rebrand the famous prison and torture chamber as a zoo. In the early years, the ignorance of the royals and their staff on how to treat the animals was something to behold.
The polar bear that swam in a sewer
A gift from King Haakon of Norway in 1252, Henry III didn't really know what to do when he received a polar bear. The bear was kept chained and muzzled for most of the time, though it was allowed into the Thames to swim, which must have been quite an alarming thing to be suddenly sprung on the fish.
Before you think the zookeepers had done one thing (vaguely) right, letting it swim was likely done as a cost-saving measure. The upkeep of a polar bear, as you'd expect, is pretty darn expensive. It's one of the many reasons why the entire world favors cats over polar bears as pets.
Though the polar bear was likely intended as a gift, large animals were sometimes given in the past as ways of bankrupting your enemies. In Southeastern Asia in the 16th-19th Centuries, kings would gift white elephants to nobles they disliked. They required a lot of care and weren't allowed to be put to work. The receiver couldn't pass the elephant on to some other poor giftee as it would be seen as an insult to a king at a time when they tended to get a bit execute-y at the slightest provocation. They would therefore be left with an elephant they didn't want and could barely afford.
The elephant that drank like a fish
In 1623, King James I received an elephant from the King of Spain. Again, this was likely meant as a genuine goodwill gesture, given the menagerie that the royal family liked to keep at the time. However, they didn't really know how to care for them and were given few instructions by the King of Spain other than a few basic (and completely nonsensical) dietary requirements.
The result was that an herbivorous elephant was fed a diet of various prime cuts of beef and a gallon of expensive red wine. The King of Spain had told James that during the winter months it could only drink wine, which they accepted as fact. For centuries, they kept giving wine to the elephants at the tower, not wondering how they came by their own bottles in the wild and apparently lacking the imagination to try offering squash.
Lions that detected virginity
Lions are not naturally found in the wet, dreary environment of the largest city in England. If they were, the English would likely lose their reputation for being polite, sophisticated, and well mannered, as having to deal with lion attacks every few minutes would probably increase their need to swear profusely.
Despite this, the Tower of London was filled with lions for many years throughout history, and they had very little chance to exercise outdoors. The little chance they were given was interfered with by being baited with dogs for the amusement of crowds, which pretty much proves our reputation for being sophisticated wasn't deserved in the first place.
One particularly grim incident occurred in the 1800s when one of the zookeepers accidentally removed the barrier that separated the lions and leopards. By the time they managed to break up the fight, a lion had already received a fatal wound.
For some reason, they also believed that lions could sense whether a woman nearby wasn't a virgin and became agitated, which probably wasn't a great thing in a deeply religious and misogynistic society, especially when the lions were basically always agitated by braying dogs.
The ostriches that ate nails
For some reason, it was believed that ostriches could eat metal. We know they look pretty hardy and their close relatives the emu have taken on the Australian military and won, but they're hardly Terminator. Nevertheless, while the elephants were eating prime rib, they were fed a diet of nails. Fortunately and unfortunately, they didn't have to put up with this diet for long. You probably won't be surprised to learn they died.