This post is sort of a hybrid Zoo History/Book Review, as most of the information I used for it comes from Daniel Hahn's The Tower Menagerie: The Amazing 600-Year History of the Royal Collection of Wild and Ferocious Beasts Kept at the Tower of London
Long before the first formal zoos were established, royal menageries were found across the courts of Europe. Some were modest collections, a few bears in a moat, others were extravagant displays featuring a wide variety of species from around the world. Of all the menageries across the continent, few had a longer life, a more colorful history, or a more dramatic setting than the one housed in the Tower of London.
When the Plantagenet King Henry III wed his sister to the Holy Roman Emperor, Frederick II, he did so with the expectation that the union would further his power and status. What he probably did not expect was that it would come with a gift of leopards - three of them (or lions... references to animals, even relatively well-known ones, in the Middle Ages can be maddeningly unclear). It isn't as oddball of a present as one might thing; throughout history, rulers have often exchanged animals between one another, such as the Zarafa, the giraffe, or Pope Leo's rhinoceros. Frederick II had a large and well-stocked menagerie, with specimens ranging from a giraffe to a polar bear, and it accompanied him almost everywhere he went. Nor where the three wild cats the first exotic animals to reach England - other kings had kept beasts for years. What was unique about these three was where they were lodged.
The Tower of London is one of the most famous fortresses of Europe, and it has served many roles over its lifetime. It has been a residence for the royal household, a mint, a garden, and the repository of the Crown Jewels. Most famously, it has served as a prison, with inmates as famous as Guy Fawkes and Sir Walter Raleigh, as well as more-than-a-few royals (Elizabeth I was herself kept prisoner here). One of its least-known roles, however, has been royal zoo.
Over the next six centuries, animals flitted in and out of the Menagerie, which grew and decayed based on the whim of whoever was warming the throne at that time. Among its residents were a polar bear who was taken to the Thames to fish for his supper every day, the first elephant seen in Britain since Roman times, what was reported to be the last British wolf, and a room full of monkeys which visitors were allowed to walk among. The most famous residents had always been the lions (actual lions, not "maybe leopards"), which held association with the British royalty. Lions and lionesses were named after kings and queens, and the death of the animal was said to forebode the death of their namesake.
Unlike many other menageries, such as the French menagerie at Versailles, the Tower wasn't limited to royalty and their visitors. It instead became one of the continent's first and most successful tourist attractions, with generations of Londoners coming to see the lions. Don't have money to see them? It's okay! Payment was also accepted in the form of a dog, cat, or other small animal to feed the caged carnivores. Soon there also arose the zoo world's oldest and most lasting April Fool's joke, "The Washing of the Lions."
The history of the Tower Menagerie was a long one, but the same couldn't be said about the animals. Conditions were poor, cramped, and dark, and animals failed to thrive in its confines. Knowledge about husbandry and care was nil, as evidenced by an elephant that was given only wine to drink and, surprisingly, didn't last too long. Even such questionable care practices paled in comparison to some of the darker whims of the Menagerie's masters. At the time of James I, bear and bull baiting were popular sports throughout England, so it made sense that royalty would have a more exalted version. During his reign, the Menagerie was reconfigured to allow fighting between lions, mastiffs, and other beasts for the king's viewing. One such dog, the sole survivor of a pack that went up against a lion, was adopted by the prince and retired from fighting on the grounds that he's faced the fiercest beast on earth and should never have to fight another.
Eventually, the ebb of time brought the Menagerie to decay and decline. By the nineteenth century, the modern zoo was already being formed across the Channel in Paris and, of course, anything France could do England had to do better. When the Zoological Society of London was established in Regent's Park (which later became the first facility to be known familiarly as a "zoo"), the remaining animals in the Tower Menagerie were relocated there. Today, the only captive wildlife left in the Tower are the ravens, the presence of which is said to safeguard the throne.
Again, for a more detailed history of the bestiary, I recommend Daniel Hahn's The Tower Menagerie: The Amazing 600-Year History of the Royal Collection of Wild and Ferocious Beasts Kept at the Tower of London, which was my main source for this article. You can find that book at Amazon.com here.