Were-worms

"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"

Were-worms were creatures of an unknown kind, possibly mythical and presumably related to Dragons, that were said to dwell in the Last Desert.

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.

Though the name "were-worm" suggests a shapeshifting creature like the werewolf of modern-day folktales, that might not be the case in Tolkien's legendarium. In Middle-earth, "Werewolves", such as Draugluin, were horrific, supernatural wolves of enormous size, but they did not shapechage. Thus Were-worms, if they did exist, could be a bizarre type of Dragon, though this must remain in the realm of speculation.