Slitherfang

Slitherfangs are mentioned briefly in several books. Specifically, the species of Burrowing Slitherfang is mentioned, however this specific species seems different than a few mentions of the unspecific "Slitherfang". Burrowing Slitherfangs are large, tentacled, underground-dwelling creatures similar to Monstrous Strangulators. The simply-mentioned Slitherfang seems to be a relatively small (perhaps the size of a large dog), surface-dwelling dragon sometimes used as a Hunting Dragon, and not mention of tentacles are made. The author most likely intended the Burrowing Slitherfang and the Slitherfang to be separate species.

In the Books
In "How to Speak Dragonese", Hiccup creates a ruckus aboard a Roman ship attempting to escape with Fishlegs and set free the dragons the Romans had captured. At one point, Hiccup used a dragon to aid Fishlegs in escape:

''So Hiccup grabbed a passing Slitherfang and shoved it up the centurion's tunic. The centurion let out a bellow and dropped Fishlegs Wouldn't you? A Slitherfang in the knickers is no laughing matter.''

The Slitherfang seen here is small, able to fit under a human's garments.

In "The Day of the Dreader", an HTTYD 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, Gronkles, Monstrous Nightmares - a multitude of dragon species."

Based on what we know about the Burrowing Slitherfang, one would not be clinging to the rafters in a Viking home, aiding humans.

In "How to Seize a Dragon's Jewel", 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.