I've read a lot about the origins of unicorn myths, the various real-life creatures they might be based on and the different cultural myths about them. What I can't figure out is where the modern depiction came from. In the middle ages, unicorns were consider ferocious beasts, wild and untamed, and possibly seen as an allegory for the Passion of the Christ and even male sexuality. But now they are associated with innocence, rainbows, and happiness. Are there any writers or artists that helped change the mythology?
In answer to a similar post, I responded today with this:
First, it is important to point out that folk legends about unicorns do not generally exist. The European peasantry simply did not believe in them and did not tell stories about encounters with them. They told stories about encountering fairies, mermaids (or seal people), and trolls, and they told about the distant past existence of creatures such as giants and dragons. But unicorns are virtually absent from material gathered from the folk.
The unicorn was an abstraction of the intelligentsia. While many fantastic creatures that appear in literature drew inspiration from the illiterate folk, a few like the unicorn were strictly literary (one often encounters exotic beasts in medieval bestiaries, but many of these were not popularly seen to exist).
Obviously reports of sightings of creatures in exotic settings that seemed like unicorns or the importation of a narwhal tusk could serve to confirm the existence of unicorns, but this would have been something entertained at court, not in the field. Consider this source for a general overview.
edit: and in answer to another similar question, I answered with the following:
It may be useful to regard the topics covered in medieval bestiaries either as being inspired by folk belief or as being taken from other, more academic sources. The illustrations of mermaids in bestiaries, for example, were taken directly from contemporary folk belief. Other creatures, including the unicorn, did not diffuse up from the folk, and in most cases, they failed to diffuse back to the folk. Because the unicorn is not a folk motif, I am not prepared to address its origin except to say that it was the property of the educated world, which borrowed material from foreign travels, Classical written sources, and who knows where (it's not my field!) to arrive at many of the creatures that occupied the pages. Volumes have been written on unicorns, but this is not my research domain because it is not a matter of folklore.
Second edit: I realize what you're after; I offer my answers to help frame the enquiry and to direct it away from "mythology" if the term is used in a folk belief sense, but rather to see that the question of unicorns is answered as a literary device.
If I may postulate here, the modern idea of a unicorn may come from biblical translation. Several passages in the Bible, and specifically the old testament use the word Re'em, which modern translations have rendered as a type of Wild ox that was rather mean spirited and rather strong.
The King James however, using the Septuigent as a guidle, translated the word the Greeks used for Re'em since there was no way for the translators to be familiar with the then rare Re'em creature. That word was monokeros, meaning "one-horned".
Those translating the King James Bible used the word Unicorn for this.
Though not as well versed in European Folklore as /u/itsallfolklore I might imagine that this had some effect.