dilligaf said:
isn't there a chance, somewhere in recorded history, that a horse or close relative (or family of them i guess..) had a birth defect causing a horn to grow from its skull?
Actually, apparently it is a fairly easy task to create a man-made unicorn by transplanting and then "re-seeding" the horn-buds of, say, a cow, so that a single horn will sprout from the centre of the animal's forehead.
In the 1930s, Dr. Franklin Dove produced just such a "unicorn cow" by the aformentioned procedure in an effort to prove that it was possible for horns to grow in the center of a forehead that had a divided frontal lobe. The cow, apparently, used the "alicorn" (the two horn buds had grown into a single horn) very differently than what it normally would have done had it had its normal two lateral horns -- it instinctively knew to charge, instead of goring, which, of course, best utilized its weight for attacking.
There's also "Lancelot" the "unicorn goat" that was once (is?) an attraction of The Ringling Bros. and Barnum and Bailey circuses which, I believe, was one of those natural birth defects (sports) that you suggested.
But as for the classic equine unicorn...is it possible to seed a horse with the horn of a cow or, even better, narwhal horn and produce the much romanticized unicorn form? You know, it's funny, I'm not sure that I've ever come across any reports of a mad scientist, throughout the annals of history, who dared to try it (and, apparently, this kind of high-tech husbandry was known from ancient times so it wasn't for lack of knowledge) and, whether it is genetically possible or not, that in itself seems just a little strange; what with the human condition being what it is and the kind of collective comfort that we would find in producing a real, living and breathing, unicorn.
Plus, as dilligaf hinted at, if humans and rabbits can occassionally sport horns, why not a horse at some point?
At any rate, all that being said, there's a neat, if slightly religiocentric, book written by Larry Brian Radka entitled
The Historical Evidence for Unicorns, that documents evidence that the unicorn was actually a distinct species of mule or ass.
Make of that what you will.
Polterdog.