* Bones
* Unicorn Horn (members only)
* Unicorn bone (during the quest Rag and Bone Man only)
* Unicorn hide (during the quest Fur 'n' Seek only)
In free to play worlds they only drop bones, However, in members worlds, they also always drop the valuable unicorn horn, and uncommonly drop lockpicks.
wilderness just below whore camp
Many places. There are some south of Varrock.
There are about 8-15 Unicorns just outside of Relleka to the East and South-east by the Golden Apple Tree.
Unicorn horns are members-only items. If you are a member, you will get horns when you kill unicorns. Otherwise you won't.
The most expensive drop you can get on runescape is the corrupt dragon battleaxe at 3.9m
bones
They will attack you, and they can drop loot. see this page: http://www.tip.it/runescape/index.php?rs2monster_id=238
No.
hill giants and hobgoblins drop them sometimes
Right click the rocks and say "drop".
To drop items, you go to your inventory, right click on the item you want to drop, then use the drop command.