When Shakespeare mentioned it in Romeo and Juliet it was already old. The proverb “a cat has nine lives, for three he plays, for three he strays, and for three he stays” is older than the USA. Nine is often seen as an magic/auspicious number in Anglo-Saxon culture, which the US is very influenced by.
The multiple lives thing goes back to the ancient Egyptians, who believed cats were divine creatures and were incarnations of the goddess Bastet (who had the power to reincarnate herself nine times). The Bastet link makes me think that nine is the "right" number of lives.


I had to look it up too. You might want to stop reading after the Kangaroo paragraph.
Kangaroos are born after less than a month, and then climb into the pouch, attach to a teat and stay there for half a year before being ready to face the outside world.
Spotted Hyenas gestate for about 110 days and then have to make it though the mother's pseudo penis, which can tear with first-time mothers (fatal to her) and can suffocate the cub (fatal to them) with cubs of first-time mums only having a 40% chance of surviving birth.