There's a long tradition of naming English/British pubs after heraldic images, mythological creatures, coats of arms (often the local landowners'), or even odd local landmarks. This became commonplace in the Middle Ages when most people could not read and therefore could not distinguish one pub shingle from another just by the lettering; however, paint a simple picture such as "The Red Lion" or "White Horse" on the shingle and people will know which one's yours and not Tom-who-sells-watery-ale down the lane.

According to a popular theory, this tradition of naming pubs after things arrived in Britain with the Romans, which is what the Inn Sign Society claims to be the case. Jacqueline Simpson's book Green Men & White Swans: The Folklore of British Pub Names is a pretty decent read, but I don't have it on hand right now to offer you quotations, sadly.

Have a look through this, too: The Wordsworth Dictionary of Pub Names for the origins of specific pub names.