The Corno da caccia is a brass instrument that comes from the group of horn instruments.
Origin: Italy/France
First used: start of 17th century Europe.
(Fit over the shoulder of huntsmen on horseback.)
Originally used for: signalling various commands to the hunting party as well as encouraging the hounds.
Began to be used in art music soon after (in compositions by Bach and Handel, among others)
Original inventor: unknown, but a modern version created by Ludwig Güttler.
What it looks like:
German Wikipedia page: https://de.wikipedia.org/wiki/Corno_da_caccia#/media/File:Corno_da_caccia_modern.jpg
Ricco-Kühn, a German trumpet-maker:
https://www.ricco-kuehn.de/1/instruments/trumpets/corno-da-caccia/
Images from a site selling a corno da caccia made in between 1900-1904:
http://www.musicantic.eu/strumenti-a-fiato-d-ottone/corni-e-corni-da-caccia/corno-da-caccia-di-pettex-muffat_4393_it_D.html
Related Instruments:
French Horn (https://musicbrainz.org/instrument/f9abcd44-52d6-4424-b3a0-67f003bbbf3d)
Bugle (https://musicbrainz.org/instrument/d8afa42b-9287-41a2-82b2-2a96cb6b6fca)
Natural horn (https://musicbrainz.org/instrument/218d072b-474a-4a9c-9d0f-c5b39891a4be)
Seems to be a subtype of the natural horn.
Sources:
https://de.wikipedia.org/wiki/Corno_da_caccia
https://www.ricco-kuehn.de/1/instruments/trumpets/corno-da-caccia/
http://www.dougmyersmusic.com/Microsoft_Word_-_History_of_the_Corno_da_Caccia.pdf
(a very detailed history!)

For future reference: https://musicbrainz.org/release/f773c777-8b2b-4878-904d-6a54a62ee6ee has a bunch of stuff that should be changed to this once it's added.