"Two lion skulls unearthed at the Tower of London have been dated to Medieval times, shedding light on the lost institution of the "Royal Menagerie".
It also shows the relationship between England's early monarchs and the "king of beasts" was not just a symbolic one."

link for you....

http://news.bbc.co.uk/1/hi/sci/tech/4371908.stm

only 2? maybe these were brought in by traders.maybe not sure no wonder why knights used lions in their coats of armor

Exotic pets are well documented in medieval times. Henry Tudor (theVIIth) had a monkey, and there are several accounts of captive lions. Some historians have postulated that the better documented English "dragon" stories could be attributed to escaped crocodiles from such menageries, though in reality, such animals would be unable to survive a British winter.

They wouldn't have to survive the winters, it's enough for a few people to see one once for the legends to begin. I mean, how often do they actually find alligators in the sewers of NYC?

i'm guessing having lions in the tower, would keep the prisoners in their cells, maybe they used them as a form of guard, but i doubt it