The "bestiaries", popular treatises on natural history which exhibit a curious admixture of truth and error, are fully illustrated in the cathedrals in the stone carving of the capitals, the parapets, and the tops of the buttresses, and in the woodwork of the stalls. For example, one readily recalls the beautiful birds of prey, the wild boars, and the feline forms of the towers of Notre Dame in Paris; the birds covered with draperies, or the elephants at Reims; the enormous oxen of the towers of Laon placed there in memory of the patient service of those animals during the construction of the Cathedral. With the animals of the country, domestic or wild, those of remote parts of the earth, known by a few specimens, are also represented. Thus we find the lion, the elephant, apes, etc.; legendary animals also, like the unicorn, the basilisk, the dragon, and the griffin.
Imaginary creatures are also frequent, and the gargoyles alone display such a variety of them as to make us wonder at the fecundity of the artists of the period. Viollet-le-Duc remarks that he does not know, in France, two gargoyles alike. These unreal figures are, nevertheless, given such a semblance of reality as to make them appear faithful copies of nature.
The failure in modern times to rival these productions of medieval sculpture, while avoiding a literal copy of them, but increases our appreciation of their value. The symbolism which usually attaches to the various animals is derived for the most part from the "bestiaries". Thus, for the lion, strength, vigilance, and courage; for the siren, voluptuousness; for the pelican, charity. The four animals which symbolize the leading characteristics of each of the Four Evangelists become more and more an accessory used to characterize the figure of the Evangelists themselves.http://www.newadvent.org/cathen/01515b.htm