Rhinobacks are Bog Dragons not encountered in the How to Train Your Dragon book series, but show up in The Incomplete Book of Dragons.
They are enormous dragons that can destroy a human settlement, both for food and entertainment. As with most dragons they are carnivorous. Rhinobacks are more aggressive than other species.
As a Viking, it pays to eat cheese—the smell of the stuff will deter them.
Rhinoback feces is predictably large, and shaped somewhat like an especially stinky cow-pie.
The Incomplete Book of Dragons also indicates that Rhinobacks can have multiple heads, and speak Dragonese.
Rhinoback remained unaligned in the Dragon Rebellion.
The Incomplete Book of Dragons, Cressida Cowell. 2014.