Slavery was a practice in the How to Train Your Dragon book series and franchise that involved a human subjucating another human or dragon to their will. Once slaves, the enslaved individual had no rights.
As revealed in How to Break a Dragon's Heart, How to Steal a Dragon's Sword, and How to Betray a Dragon's Hero, Grimbeard the Ghastly, King of the Wilderwest enslaved both humans and dragons to build the City on Tomorrow.
Around a century later in How to Ride a Dragon's Storm, the Hysteric Tribe captured several Northern Wanderers during their raid on their village in the Arctic. These slaves were kept on The American Dream 2 for the purpose of rowing. Eventually though, the Wanderer slaves were freed by Hiccup Horrendous Haddock III.