Burrowing Slitherfangs are mentioned four times in the book series, once in How to Be a Pirate, and again as a dragon species profile at the end of A Hero's Guide to Deadly Dragons, in How to Steal a Dragon's Sword, and finally they are referred to in How to Seize a Dragon's Jewel.
There is no complete depiction of a Burrowing Slitherfang, save for its numerous tentacles. They can become quite large.
Burrowing Slitherfangs have not been assigned a class. Categorized as a Cave dragon is a possibility because it's a relative the Monstrous Strangulator and can be classed as such. They are also mentioned in How to Steal a Dragon's Sword when Hiccup describes the possible creatures they might meet in the caves underneath Flashburn's School of Swordfighting:
|“||And then there are Breathquenchers - a bit like gigantic pythons - and Burrowing Slitherfangs, of course . . . and Stickyworms||”|
|— (Book 9)|
A Sea Dragon classification is also possible, as they are frequently eaten by Seadragonus Giganticus Maximus, a Sea Dragon. The Monster of the Amber-Slavelands is also similar at least in behavior and habitat, and this dragon is likewise classed as a Sea Dragon. In fact, Hiccup uses his knowledge of Burrowing Slitherfangs to overcome the Monster of the Amber Slavelands inHow to Seize a Dragon's Jewel.
Dragons referred to generically as Slitherfangs are mentioned throughout the Book series, however, never in the context of the specific information given for the Burrowing Slitherfang. Most likely the Burrowing Slitherfang and the generic "Slitherfang" are separate (but possibly related) species.
Burrowing Slitherfangs live under unstable "fluid" ground such as quicksand and bogs. They use their tentacles to grab its prey at the surface, then drag them down underground to suffocate before they are eaten.
Their main diet is composed of other dragons—particularly Bog Dragons—and the occasional Viking. Because the victims are taken underground, it is extremely difficult to find any trace as to what happened to them.
As Hiccup navigates the caves underneath the Isle of Berk, he discovers a skeleton.
|“||However, even the discovery of an entire skeleton of the Burrowing Slitherfang, so rare that it was often thought never to have existed||”|
|— Book 2|
The Burrowing Slitherfang does not appear in the story of this book, but does appear in a species information page at the end.
When Hiccup and Fishlegs must enter the caves beneath Flashburn's School of Sword Fighting, Hiccup tells Fishlegs that they may encounter Burrowing Slitherfangs, among other species.
As Hiccup tries to escape the Monster of the Amber-Slavelands, he draws upon his knowledge of the Burrowing Slitherfang to help defeat it, as they have some similarities.