I don't know abour Sigourney Weaver's cat in "Alien", but the cat in "Bell,
Book, and Candle" was called Pyewacket.  This I know because when I was a
teenager my family had a cat wheo was named Pyewacket after the one in the
move.  Good old Pye, she lived to be 23 years old.
As for other 'famous' cats, don't forget DC in "That Darn Cat", Thomasina in
"The Three Lives Thomasina" and Cosmic Creepers  in "Bedknobs and
Broomsticks!"