This is a post horn (sometimes spelled posthorn, sometimes called a coach horn).
Question: The post horn is usually coiled copper or brass, rarely straight. It has no valves or slide, and was originally used to announce mail coaches. The Post Horn Serenade is the nickname of Mozart’s Serenade #9, simply because he used the instrument in it.
As far as I can tell, the post horn has been used in only a couple of other pieces by composers. One example is the Post Horn Polka that for a long time was attributed to (the obvious choice for a polka at that time) J. Strauss, but may have been composed by Louis Antoine Jullien, a French conductor. The only other composition that is that well known today that uses the post horn is … what?
Answer: Mahler’s Third Symphony has an off-stage post horn solo.