Romans brought elephants to England during their invasions in first century of the common era, but after that, more than a thousand years passed before another elephant was actually seen by people in Britain- or most of Europe. But travelers brought back tales, which artists tried to illustrate. For his masters thesis at the University of Arts in Berlin, Uli Westphal created a taxonomic tree of those illustrations called Elephas Anthropogenus. It was later published in Zoologischer Anzeiger - A Journal of Comparative Zoology.
Since there was no real knowledge of how these animals actually looked, illustrators had to rely on oral, pictorial and written transmissions to morphologically reconstruct an elephant, thus, reinventing the image of an actual existing creature. This led, in most cases, to illustrations in which the most characteristic features of elephants – such as trunk and tusks – are still visible, but that otherwise completely deviate from the real appearance and physique of these animals. In this process, zoological knowledge about elephants was overwritten by its cultural significance. Based on a collection of these images I have reconstructed the evolution of the ‘Elephas anthropogenus’, the man made elephant.
Westphal's interactive chart is posted here. Click on an elephant to see the drawing up close. There are also links to the originals, like the hoofed elephant from the year 1444 shown here, although many suffer from link rot. -via Metafilter
Update: You can see quite a bit of those medieval artworks in a gallery at Flashbak.