Very little description is given, but the sketch in side of Book 11 shows a four-legged dragon with a stubby, short-snouted head. As it is a riding dragon, it is large enough to carry at least one human.
Though not specifically stated, it might be assumed Hurricanes are sky dragons because "Luckily the Windwalker, the Deadly Shadow, and the Hurricane were some of the fastest riding dragons in the Barbaric World - only a Silver Phantom flies faster."
In the beginning of the novel, Hiccup found "lying on his front on the island of ice was a human, chained to a sleeping riding dragon, a Hurricane, with scars and whip marks all along its side." This is revealed to be Snotlout.
Snotlout's Hurricane makes an appearance again after Snotlout and Hiccup are escaping from Alvin's War Bunker with King's Lost Things. However, nothing is said about what happened to this dragon by the close of the book.