Cat gut

Quote from Wikipedia.  Does not mention cats...

Natural gut is produced by drying fibers extracted from a part of the cow intestine called the serous membrane, or serosa, which contains collagen designed to withstand the stretching and contraction of the intestine. It is this elasticity that makes the fibers useful for tennis string. Sheep intestines have also been used for racquet strings in the past.^[5] [6]

The first natural gut tennis string was rumored to be manufactured in 1875 by Pierre Babolat, who would launch the VS brand of gut fifty years later. Natural gut is usually offered in coated form, to reduce its tendency to unravel, particularly when humid or wet.