Wikipedia says: Catgut is a type of cord that is prepared from the natural fibre found in the walls of animal intestines. Catgut makers usually use sheep or goat intestines, but occasionally use the intestines of cattle, hogs, horses, mules, or donkeys. Despite the name, catgut manufacturers do not use cat intestines.
For a long time, catgut was the most common material for the strings of harps, lutes, violins, violas and cellos, acoustic guitars and other stringed musical instruments, as well as older marching snare drums. Most musical instruments produced today use strings with cores made of other materials, generally steel or synthetic polymer. Gut strings are the natural choice for many classical and baroque string players, and gut strings are still most commonly preferred in concert-tension pedal/grand and some lever harps because they give a richer, darker sound as well as withstanding high tension within low alto, tenor, and high-bass ranges. Acoustic guitars moved away from gut strings in the early 1900s when the C. F. Martin & Company introduced steel strings. These gave greater volume to the guitar. “The demand for steel came from ensemble players, who couldn’t make themselves heard clearly without it.” Within a few years the majority of Martin guitars were made with steel strings to accommodate the demand.