- "Tell me what you want done, and I will try it, if I have to walk from here to the East of East and fight the wild Were-worms in the Last Desert."
- ― Bilbo Baggins, from The Hobbit, "An Unexpected Party"
Tolkien only ever mentions Were-worms once, in the quote given above, so we know almost nothing about them. We cannot even be certain that they actually existed — the Hobbits had a rich folklore full of fantastic creatures, and Were-worms quite possibly fall into that category.
The element "were-" is a Germanic term that refers to humans. It suggests a shapeshifting creature like the werewolf of modern-day folktales. In Middle-earth, "Werewolves", such as Draugluin, did not shapechange and the name perhaps refers to the size and intelligence.
Thus Were-worms, if they did exist, could be a human-like type of Dragon, though this must remain in the realm of speculation.