U.S. Patent No. 4,429,685 (.pdf)
I thought that this patent would delve into elves, warlocks, and some druids, but lo and behold, it's rather educating! As it turns out, horn buds aren't initially attached to the skulls of new-born cattle/antelopes/sheep/goats. For a week they grow in the skin, with no connection to the bone. That gives you seven days to whip out the scalpel, do some incisions following the origami-esque diagrams, stitch everything back up, and now you've got mythology growing right in front of you. As a bonus, your surgery will "render a unicorn of higher intelligence", although, as is noted later, that conclusion hasn't been proven (see col. 3, line 41).