Medieval art, literature, folklore and imagination was populated with various anthropomorphous creatures, beasts, sirens, werewolves, demons, dragons, griffins, hybrids, ogres, vampires and numerous others, which reflected a fascination with unusual, challenging and anomalous bodies.
If you are interested in this imaginative world of wondrous beings, take a look at the British Library's Medieval Monsters illustrations accompanied by short descriptions as well as an interesting slideshow. These images reflect the wide variety of creatures that populated the medieval imagination and offer a glimpse of the rich and varied monstrous embodiments of the period.
Here is an illustration from the site featuring sirens, accompanied by the following text:
In most Bestiaries, these animals are interpreted in relation to Christian morality: the creatures themselves were not as important as the moral truths revealed in their explication. Sirens, for instance, were said to have the upper body of a human and the lower body of a bird or fish (or even a combination of the two); they sang beautiful songs to lull sailors to sleep, and then attacked and killed them. The moral: those who take pleasure in worldly diversions will be vulnerable to the devil.