Slitherfangs are mentioned briefly in several books in the How to Train Your Dragon Book series. Specifically, the species of Burrowing Slitherfang is mentioned, however this specific species seems different than the few mentions of the unspecific "Slitherfang".
Burrowing Slitherfangs are large, tentacled, underground-dwelling creatures similar to Monstrous Strangulators. The simply-titled 'Slitherfang' seems to refer to a relatively small (perhaps the size of a large Dog (Books)|dog]]), surface-dwelling dragon sometimes used as a Hunting Dragon by Vikings. No mention of tentacles are made. The author appears to have intended the 'Burrowing Slitherfang' and the 'Slitherfang' to be separate species.
|“|| So Hiccup grabbed a passing Slitherfang and shoved it up the centurion's tunic. The centurion let out a bellow and dropped Fishlegs |
A Slitherfang in the knickers is no laughing matter.
|— How to Speak Dragonese|
The Slitherfang seen here is small, able to fit under a human's garments.
In this How to Train Your Dragon novella, there is mention of Slitherfangs as hunting dragons:
|“||Up in the rafters perched the hunting-dragons, large as crocodiles, their ribs showing through their skin, their eyes watching the cheering Vikings below. There were Slitherfangs, Gronckles, Monstrous Nightmares—a multitude of dragon species.||”|
|— The Day of the Dreader|
Based on what we know about the Burrowing Slitherfang, one would not be clinging to the rafters in a Viking home, aiding humans.
Beware Slitherfangs is written on the island of Swallow, which appears at the beginning of this Book.
In Book 10, Slitherfangs are mentioned as being hazardous underground creatures and possibly being related to the Monster of the Amber-Slavelands, but this most likely is referring to the Burrowing Slitherfang.