Medieval bestiaries and folktales frequently feature bizarre and implausible creatures. This apparent willingness to accept the existence of any sort of monster is the product of two important trends in medieval writing.
Travel over long distances in the Middle Ages was extremely difficult and so most people were left to rely on the reports of those few of their contemporaries who had the time and resources to do so, coupled with reports handed down from antiquity.
The travellers were often unable to properly relate what they had seen since they had to explain it to their non-traveling friends back home. Consequently, descriptions tended to be imprecise and caricaturish.
This exageration produced outlandish creatures like the Vegetable Lamb of Tartary. In reality Tartary was home to a plant with white flowers which resembles a sheep at distance. In the medieval imagination this eventually became a half-plant half-sheep creature.
The classical narrators were no better than the medieval ones. In particular, Pliny the Elder’s natural histories accepted almost any reported animal, which led to a seemingly authoritative Roman text attesting confidently to the existence of manticores and basilisks.
The real focus of medieval descriptions of beasts, though, was not to catalogue existing animals. The main function of bestiaries and other texts on animals was to present moral or spiritual ideas figuratively.
Some animals were more symbolically laden than others and just because an animal might be more fantastical than another that needn’t indicate it was also more symbolic.
The unicorn is typical of the kind of symbolic writing which characterised medieval approaches to animals. It was used to represent Jesus; the single horn represented the unity of God and Christ within the holy trinity with the unicorn’s traditional small stature representing humility.
The legend that only a virgin could capture the unicorn also contributes to their Christ-like presentation, recalling both a generalised notion of purity and his connection to the Virgin Mary.
Another example of this is St Christopher, who has sometimes been shown as a dog headed giant since the Middle Ages. This partly arose due to similarity between the word canine and Christopher’s homeland of Canaan. The dog-head myth was also used to stress Christopher’s uncivilised nature before converting to Christianity. In one version of the legend he actually transforms from dog-headed to human-headed after proving his holiness.
In common with other fantastical elements of medieval world views this fascination with monsters and magical creatures had little to do with observing how the world worked, but rather expressed a particular understanding of how the world ought to work.