The corno da tirarsi is a Baroque brass instrument. It appears in four Bach cantatas. Its characteristics are debated, but it was most likely a horn that resembled the "Inventions" horn of the late 18th century: "a freely-moving, double (cylindrical) tuning slide positioned diametrically across its circular body". It may also have been a slide trumpet.
References
Corno da tirarsi Wikipedia(Text) CC BY-SA