My mistress has always called me pet, rather than slave. I've always felt like a pampered little kitty with her, she's always taking care of my needs, and doesn't really order me around very much. A lot of people look at their pets (animals) as their best friends, but whom they are still the boss of at the end of the day, and I think our relationship is a lot like that.

That being said, another person's definition of pet, or slave, or even submission could be completely different than mine or yours. Referring back to your original post, you asked if the term "pet" might fill some sort of gap, but rest assured that all of the gaps are already filled, even if just by different ways of using the same words. ^.^