The Grimwing was mentioned once in How to Steal a Dragon's Sword. It was part of the group of Red-Rage dragons attacking Hiccup and the other young Vikings on the cliffs at the Gorge of the Thunderbolt of Thor. Other dragons that were part of the attack were: Razorwings, Doldrums, Tonguetwisters, and Vampire Ghouldeaths.

"One hunting dragon went down in a blaze of flames, a Monstrous Nightmare belonging to Very Vicious the Visithug that sank its teeth into the neck of a much larger Grimwing, which then grabbed it around the throat, and snapping, biting, screaming down they went, doomed, into the Gorge in a somersaulting ball of fire."
How to Steal a Dragon's Sword

There is no description, other than what might be inferred—that it is a largish dragon, possibly a Mountain Dragon like the Razorwing and Tonguetwister it was accompanying, and had a strong dislike for humans.