Black Unicorns are a a great way to make money on Runescape. The Black Unicorns drop Unicorn Horns which sell for quite a lot on the Grand Exchange although the prices go up and down a lot. I’ve seen them as high as 5000 and as low as 1000 gold each. Regardless there is money to be made killing Black Unicorns, and you level up combat skills at the same time.
I prefer to farm the Black Unicorns in the level 10 – 17 Wilderness, usually there aren’t anyone around to PK you here anyway so it is pretty safe. They can be found just southwest of the Wilderness Volcano. Just farm these until your inventory is full, go bank or sell and come back. Rinse and repeat and you should have a pretty nice hourly profit on these.