better beasts... the Ashmole Bestiary

The Ashmole Bestiary, held in the Bodleian Library, London, is a late 12th century or early 13th century illustrated text packed with allegorical descriptions and depictions of beasts, birds, fish, and insects. It also includes such mythical creatures as the Bonnacon, or the Bonasus (the last in the series of illustrations above), whose volcanic tush is to be much-feared. Pliny, in his Naturalis Historia, explains it thus:
There are reports of a wild animal in Paionia called the bonasus, which has the mane of a horse, but in all other respects resembles a bull; its horns are curved back in such a manner as to be of no use for fighting, and it is said that because of this it saves itself by running away, meanwhile emitting a trail of dung that sometimes covers a distance of as much as three furlongs [604 m], contact with which scorches pursuers like a sort of fire.
Medieval art is renowned for its schematic rather than naturalistic depiction, which most scholars agree fits with a spiritual rendering. Illustrated texts of this period placed more emphasis on spiritual-symbolic-realism rather than terra-scientific-realism, which is why Our Heavenlies appear larger than their mortal counterparts. Suiting formal planar devices – boundaries, positions, directions, proportions, and perspectives – to a spiritual and moral doctrine was commonplace. Whilst medieval artists and patrons of the arts often expressed admiration for classical sculpture's likeness (and sometimes improvement on) the 'real thing,' scholars and ofttimes-pesky men of the church believed it an offence to God to make verisimilitudes. Augustine of Hippo and others like Meister Eckhart agreed that a naturalistic art, made "according to the eyes and only for the eyes"†, was an attempt to rival the Creator, and anyway, would not succeed because mere man could only get as close as a substandard copy (which, it must be pointed out, would rile the Almighty even further).

Hubris and poor imitation aside, images were problematic for another reason. As the literature of the illiterate, the poor read in them what they could not read in books, which made them potent aides to doctrine. The fear, of course, is that they may be too potent and become objects of worship themselves. Iconoclasts, who sought to end the idolatrous veneration of images by destroying said images, could sometimes be placated by representational theories; some Byzantine scholars talked of the space between the paint and the viewer as being occupied with spirit, rather than something in the material itself, and certain medieval scholars downplayed the importance of the image to such a degree that it apparently held no power at all. The Libri Carolini, the middle age's bible (pun apologies) of representational aesthetics goes so far as to declare that a statue of Venus and the Virgin are formally and materially equal, but it is their inscriptions that differ† – and this is what matters. Whatever the case, the beasts of Ashmole are all the better for it.

† 'Meister Eckhart's Views on Art' in The Transformation of Nature in Art (1956)

† Book IV, chapter 16