Remarkable instrument from the 1955 children's book by Dr. Seuss, On Beyond Zebra!. It combines a horn with an accordion bellows and reed mouthpiece, as well as several switches somewhat resembling organ stops. It's described as quite heavy and difficult to learn. Despite this, it is a "hunting horn," as a properly played o'Grunth is the only thing that can lure a Flunnel out of its burrow.
As given in the text, the "o" in the name is properly in lowercase.