General Method Description

Upon completion of the quest The Tower of Life, you gain access to a special feature that allows you to generate oddball creatures. You put items from two different monsters on an altar and then bring to life a creature that is a bit like each of the monsters its items came from; when you kill the monster it drops several of one of the items. Most of these are not very useful, but one is: the unicow. From one unicorn horn and a cowhide you create this monster, which drops 2 to 4 unicorn horns when it dies. Because all other methods of getting unicorn horns are so slow, this turns out to actually be the fastest way in the game to get them!

First of all, this is combat so you'll want to put on normal combat gear. You also need to bring one unicorn horn and a bunch of cowhides; you get more horns with each kill but the hides are used up. The Ardougne Achievement Diary provides rewards that are particularly helpful for this activity. First, completing the easy tasks gives you a cloak that will teleport you to the monastery south of Ardougne, shaving 30 seconds off each trip to the Tower of Life, as well as saving you a bit of money. But more importantly, wearing any Ardougne cloak causes some of the unicorn horns dropped by unicows to be noted! This allows you to stay longer on each trip, letting you get more horns per hour. The better the cloak, the more of the horns that drop in noted form. There is one drawback to this, however -- you have to be careful to ensure you don't run out of unnoted horns to operate the creature creation machine. This can be done by removing the cloak occasionally, as I describe in the specific method instructions for each method level.

Incidentally, I have also tested killing regular unicorns and also black unicorns in the Wilderness, and this method is much faster than the former, and a little faster (and much safer) than the latter.