The Dreamserpent is a dragon that is only mentioned in How to Speak Dragonese.
The Dreamserpent is neither seen or described in the Book series. It is possible this dragon has a serpentine body shape, based on the "serpent" portion of its name.
Its feces, however, is depicted in The Incomplete Book of Dragons. They can "drift through the air, as light as cobwebs."
Dreamserpents are listed as Tree Dragons in the back of The Incomplete Book of Dragons, in the Key to Dragons. The Key shows that they can speak both Norse and Dragonese. They also possess chameleon abilities (its not clear if this means they can change colors, or simply blend in with their environment). In addition, Dreamserpents have the rare ability to look into the future. Only Horrors and Seadragonus Giganticus Maximus dragons are known to be able to do this.
Only the Dreamserpent's feces is described in this reference book, as well as appearing in the table at the back of the book.