post horn

from The Century Dictionary.

• noun A postman's horn; a horn blown by the driver or guard of a mail-coach, and at present used on four-in-hands for pleasure driving.
• noun Same as bugle, 1.
• noun A musical phrase or call, such as is given by a post-horn or bugle.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun historical A type of brass horn used to signal the arrival or departure of a post rider or mail coach.
• noun music A valveless musical instrument based on such a horn, used in orchestral music.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun wind instrument used by postilions of the 18th and 19th centuries