Horn probably was one of the very first musical instrument. It was originated form the animal horns. However, there is no evidence of when was horn being developed from animal horns to metal-made horn.
Many of you might wonder why bell of modern horn is facing backward. The reason is, around 15th or 16th century in Europe, horn was being used as a signal for hunting sport. This signal needed to be given to king or royal family backward. This was why horn bell is facing that way. Because of this reason also, horn that was used for this was called 'hunting horn'.
Here are some interesting YouTube videos of hunting horns