Bullingdozers are not seen in the book series, but are mentioned in the Dragon Key at the back of The Complete Book of Dragons. They are classified as Tree Dragons that are very large, perhaps along the order of Darkbreathers, Battlegores, or Bee-Eaters (all of which received 1 'dragon-next-to-a-mountain' icon). They are noted as possessing multiple heads. Bullingdozers can speak Dragonese, and interestingly, have hypnotic capabilities. They remained unaligned during the Dragon Rebellion.
There is no imagery of the Bullingdozer, but we do get to see what this dragon's feces looks like:
- The Complete Book of Dragons: A Guide To Dragon Species, Cressida Cowell. 2014