[YogicCowboy]

Yes. I think Edgar Rice Burroughs wrote books in that vein (Mars). I only read Tarzan and some Pellucidar.

[Harmless Teddy Bear]

In David Webber's "Honorverse" an entire arc is devoted to a company called "Manpower" that deals in genetic slavery.

Heinlein's books often include slavery in the future.

Alan Dean Foster Commonwealth books also have slavery.

I would say it is almost as common a plot line as war.