Unlike the previous poster, I wouldn't be too certain about tracing the arms before Henry ii, but in any case the question has been asked a couple of times before, so here's the link to the faq. The tldr of most of the threads was that lions were mostly known through Classical literature, and the bible.

Edit: Not all that relevant, but I like posting this link: Medieval depictions of lions are often fairly inaccurate, but at least they had fairly close models to base it on. However, their elephants tend to be entertainingly bad.

If I remember correctly it had been the coat of arms of the Duke of Normandy, and when William conquered England in 1066, he transferred the Lion to the English banner.