"To tell the truth, he had felt sadder when he had to put down one of his favorite Rottdragons who had been injured in a fight."
Grimbeard the Ghastly

Grimbeard the Ghastly showed little remorse when abandoning his child, Hiccup II. But he seemed to have a fondness for the species of dragon called Rottdragons. This is the only mention if this species, and no further information is given. It might be extrapolated from its name that it mimics the dog breed the Rottweiler in appearance or function. As such, the Rottdragon was possibly a type of hunting dragon.