|“||Sadly, Bertha, Chief of the Bog-Burglars, sharpened her axe, looking back at happier times when she was striding waist-deep in the delightful bogs of home, her faithful Goreblaster swimming by her side.||”|
|— How to Fight a Dragon's Fury|
There is no description or further information about the Goreblaster.
The above passage implies a Goreblaster might be classed as a Bog Dragon, and used as a Hunting Dragon. Because the dragon is swimming while the human is wading through water, it is probably around the size of a dog.
The Goreblaster is only mentioned once in the final Book of the How to Train Your Dragon Series.