Were-worms were most likely imaginary or mythical creatures told of in the stories of hobbits. But then again, most stories have some grain of truth in them. The were-worms were supposedly terrible monsters that made their home in the Last Desert very far east of the Shire. For a hobbit to declare himself prepared to fight were-worms was a sort of proverbial way of saying that he was ready to do anything.
"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, The Hobbit
That is what I was thinking because, were-worms as in werewolves. Shape shifting in a way. I think they are what you say they are, that is what I think anyway.
That is what I think. And they have the aperence of a huminoid dragon. But I also think it may be posible that they have long bodys like Runic dragons or Wryms. And they swim in the sand and pop out and eat their pray.