Serpent-tongues are mentioned once in How to Seize a Dragon's Jewel, at the commencement of the battle at Prison Darkheart.
- "Serpent-tongues writhed and twisted around the white dragon bones, shaking out their wet wings. . . . The darkest types of dragon species, the monsters, the ones that hated humans the most."
- ―How to Seize a Dragon's Jewel
These dragons are not mentioned again. There is no further information about them. They obviously have (presumably functional) wings, and live under the sands of the Amber Slavelands. The verbs 'writhed' and 'twisted' imply a serpentine body shape, which might be on the smaller end of the dragon size spectrum, since Serpent-tongues are twisting about other dragons' bones.
Serpent-tongues are clearly part of the Dragon Rebellion, against the humans.
- How to Seize a Dragon's Jewel (Book 10), Cressida Cowell. 2012