A Dimbruiser is a dragon of mentioned on the official How to Train Your Dragon Books Website.
There are no written descriptions or imagery of this dragon. Because the Dimbruiser is mentioned as a riding dragon, it is large enough to carry at least one adult human and sustain flight.
The budding Viking Clueless has one as a riding dragon. Not much is known about this species. Because Clueless' low intellect is highlighted in the Book series, and his Hunting Dragon, a Lackwit, is noted to be unfocused, it might be assumed that the Dimbruiser is likewise, not very bright ("dim") and/or rather bullish in nature (a "bruiser").