Eight-legged critters and cats originate from outer space. I have my suspicions about six-legged critters, too.
Had a spider come up through a miniscule crack in the floorboards of my bathroom, decided to watch her/it/him while she/it/he built a web around the crack, about 3-4" out from the floorboard. Named her/it/him "Abigail," true gender notwithstanding.
Observed for a little more than a week. Would scuttle back into the crack if I stomped the floor. Twice I dropped a recently-deceased but still intact fly on the web. Boy, was she/it/he fast! I was amazed.
Accidentally and carelessly drew her up into my vacuum cleaner.
End of experiment.
So I guess you could say I kept a spider as a pet.
Sealed the crack with silicone.