The Bullpuff is a hunting and lap dragon mentioned in The Incomplete Book of Dragons.
The Bullpuff is shown along with Puggles and Specklehounds which are classified as Mountain Dragons, so perhaps Bullpuffs are, too. Both the aforementioned dragon species bare resemblance to certain dog breeds - Pugs and Dachshunds, respectively. The Bullpuff seems similar to a small bulldog - rather squat, compact, and large jowls.
The Bullpuff is not included in the Dragon Key at the back of the book, so a class or additional information is not known.
Though the reader is not given a numerical designation of "Disobedience", and therefore trainability, the Bullpuff is trainable as seen in the image above. Bullpuffs are small dragons used by Vikings as pets and hunting dragons.
The a picture of a Bullpuff only appears in this reference book, with little other information. The Bullpuff illustrated is named Crashbark, a pet dragon of Hiccup's Aunt Egginbreeza Six-pints.