He knows scattered bits of the human history of his world, thanks to the trash that gets underground. It's not exactly comprehensive, but the notion of ownership has come up before, and Papyrus shivers as he looks again at the monsters around them.

"You think it's sss... People owning other people? Treating people as nothing but pets, all the way...?"

Sometimes the lines are blurry, in an underground full of monsters and animals of all shapes and sentiences. That annoying dog is smart and tenacious, maybe more a pet than a person, but not clearly either. One of the rabbit families in Snowdin has a young quadrupedal kid, whose older sister takes him out on walks with a leash... the better to keep him from hopping off a steep cliff, or doing his best to get in a tree, or something. And some spiders are people, some spiders are pets, and some spiders are food... and with how private spiders can be, only the spiders themselves really know the distinctions.