A gemshorn (one word) is the horn of the gems (hard ‘g’). A gems is an Alpine mountain goat, which had holes drilled in it to make a wind instrument. (That’s the horn, not the goat. And best if you detach it from the goat first.) The open end is plugged with a fipple and a lip is made to blow into. Nowadays you can’t get gemshorns, so the horns of other suitable cattle are used, such as cows.
There is very little evidence for the gemshorn. One of the few references is in Musica Getutscht und Ausgezogen, published in 1511, by the German musician Sebastian Virdung, who described it as a short, curved animal horn, with four finger holes. (An instrument very much like a gemshorn was found buried underneath the foundation of a house built in 1450, but that was made of clay.) Another German musician and documenter, Michael Praetorius, gives gemshorn diagrams in his De Organographia, 1618. So the horn was played from the mid-renaissance to early baroque, but before or after that it doesn’t appear in any known documents.
The renaissance gemshorn had four finger holes, but modern makers tend to drill seven holes in the front and one in the back, which gives the fingering of a recorder and increases the range of possible notes.
Gemshorn playing Packington’s Pound, anon., England, late 16th century