Enbrethiliel has a good post up about animals and God. (It must be good; she links to me.)
It is lucky for dogs that we Catholics accept Jewish scriptures written in Greek — because Jews in Israel didn’t think much of dogs, but the writer of the Book of Tobit thought that of course Tobit should have a dog. So if you’re Catholic or Orthodox, pets are scriptural. If you’re not, you have to rely on human tradition.
Here’s an interesting article on the Book of Tobit in the Dead Sea Scrolls and St. Jerome’s tail-wagging addition.
Cats have been important helpers of humanity for a long time, and Pope Benedict XVI isn’t the only churchman who’s liked them. For example, there’s the 9th century Irish monk at Reichenau who wrote a little poem about his cat, White Pangur (Pangur Ban). (More translations.) Apparently Pangur Ban (along with his monks) is a main character in an old kids’ Celtic fantasy series by Fay Sampson. Looks interesting.