The Black Unicorn drops Unicorn horns which it can be sell in Grand Exchange which it can be a great source to sell RS gold.
Source: Wikia
Making RS gold by gathering Unicorn horn
Black Unicorns are the opposite of regular unicorns which is a bit more dangerous and aggressive version of unicorns which can be found in the Wilderness. Hunting Black Unicorn as more effective for gathering Unicorn Horns rather than the regular unicorns which they have greater numbers, it can be a great source to sell RS gold. They also have faster spawn rate and you can found them around the area of Wilderness. Be wary that once their health points are too low, sometimes they tend to run away. Below are the drops of Black unicorns and their RS gold price:
Source: Wikia
It is preferred to farm Black Unicorns in the level 10 to 17 Wilderness which is to say they are safe against PK players. Black Unicorns can be found southwest of the Wilderness Volcano. Kill all the Black Unicorns you can find and fill up your inventory with it (if you have Beast of Burden, then use it and fill up the inventory). Sell those Unicorn horn at the Grand Exchange for RS gold.