JF Ptak Science Books Post 1898
Who could refuse to see a medical person advertising themselves as:
“the Doctor Liveth in Houndsditch, next Door to Gun-Yard, having a Back Door into the Yard, where any Patient may come…”
and especially one selling Unicorn horn curative extract? The cure promises of “the beast that no existence hath” (Rilke, Sonnets to Orpheus) sounded attractive, having the ability to dispense “scurvy, old Ulcers, Dropsie, Running Gout, Consumptions, Coughs, palpitations of the heart, Fainting Fits, Convulsions, King’s Evil, rickets in children, melancholy or sadness, the green sickness, and all distempers proceeding from a Cold Cause” as well as “all crudities in the Body”. But promises like this are almost always loyal only to that pretended self.
[Source for both images: Rudiger Robert Beer, Unicron, Myth and Reality, Mason Oress, 1977.]
In spite having a very long sales history over the milennia, pretty much the only place where unicorn horns have actually been seen were in apothecary shops selling narwal horns and in the constellation Monocaros (named so for the unicorn by Jacob Bartsch around 1624). This great ground-up thing, one of the great mythic myths of all time, was a highly-inclined multi-purpose curative purgative suggestative ill-spoiler, capable of defeating nasty bodily nasties and enhancing things like our Noble Parts. For years it was worth more by weight than gold, though (depending on what it was that was being sold as unicron horn) ingested gold might have had about the same medicinal effect as a dose of unicron horn.
This fabulous image of a unicorn, draped in Renaissance-y literary moudlings appears in The secrets of Albertus Magnus: of the vertues of herbs, stones, and certain beasts : whereunto is newly added, a short discourse of the seven planets, governing the nativities of children : also a book of the same author, of the marvellous things of the world and of certain things, caused of certain beasts being a translation of De virtutibus herbarum, lapidum, & animalium quorandam libellus, and his De mirabilibus mundi, ac de quibusdam effectibus causatis à quibusdam animalibus (1543).
Hope faded in the greatest degree for the unicorn only in the 18th century, replaced immediately by any other number of empassioned impossibles, like magnetisma and curative electricity.
A few handy resources:
http://www.storehousesoflearning.com/links.htm Useful links for Medieval images and manuscripts
http://bestiary.ca/beasts/beastalphashort.htm Great Bestiary, with searchable index of creatures; once inside, there are references to the animal from some of the leading Medieval bestiaries (and their ancinet sources), including Herodotus, Strabo, Guilaaume le Clerc, Pliny, Isidore, John Mandeville, and others.
Medieval book of birds http://archive.org/stream/medievalbookofbi00hughuoft#page/n7/mode/2up
These elements seem to be most populous in the illustrated sections of early encyclopediae, and dictionaries, and even encyclopedic dictionaries, where a number of different elements are displayed on the same page, different and generally unrelated images on the same engraved sheet, references for articles found in different parts of the book. 
