Cats are an interesting point to the comparison of pets to slavery.
How could one argue with outdoor cats as pets? They are not held against their will; they leave frequently and return according to their own choice.

As for other animals as pets like dogs, indoor cats, or monkeys, what makes it okay to keep them as pets but not okay to keep humans as pets?
Is it intelligence? Does that mean it is okay to keep stupid or retarded humans as pets?

I have no answer to these questions, this is one of those places where western society is a googol of contradictions.