Charming Horn is one of the woodworker's crafting item for War Horn, Exquisite War Horn. There are numerous forum threads with people asking where it comes from, but no answers.
I can help you with this one. There is a known issue with the drop rate of the Horn. It is effectively 0.
For Volume 2, Book 7 the item is being changed to an Elegant Claw and will begin dropping correctly.
Sorry for all the frustration and confusion.