According to this 1940’s ad:
“Mark loved cats…and pool, or pocket billiards. Whenever he played pool, he’d tuck a pet kitten into a corner pocket. The antics of the cat as it pawed at passing balls amused and relaxed Mark Twain…sent him back in high spirits to write the stories that endeared him to all.”
Via Mark Twain Quotations about cats, where you’ll find several more images of Twain with his cats.