Tonguetwisters are mentioned in several books, but most notably in How to Steal a Dragon's Sword.
Tonguetwisters are larger size dragons with multiple eyes, as evidenced in their pictures. They have long spikes along their spines. One images depicts them having a green hide and pink colored spines and horns. This dragon's signature trait is its long, muscular, and flexible tongue.
Tonguetwisters are able to rend prey and enemies from limb to limb with their tongues. They have acrid black saliva, as many of the dragons do. They have They are also said to have X-ray vision in The Incomplete Book of Dragons.
Tonguetwisters are able to speak Dragonese.
Hiccup and the other Viking novices scale a shear cliff on their way to Flashburn's School of Swordfighting, and spend the night in hammocks. During the night, Red Rage dragons attack the boys, including several Tonguetwisters.
Tonguetwisters are seen in this reference book in several sections, as well as the 'Dragon Key' at the back of the book.