See the below entry from Wikipedia:
Catgut is the name applied to cord of great toughness and tenacity prepared from the intestines of sheep/goat, or occasionally from those of the hog, horse, mule, and donkey. Those of the cat are not employed, and therefore it is supposed that the word is properly kitgut ("violin string"), kit meaning "fiddle," and that the present form has arisen through confusion with kit = cat. Another explanation of the origin of the cat in catgut is that it is an abbreviation for cattle which originally denoted not only cows, but all types of livestock.