No written description is given for the Sand Rattler. In the "Viking Dragons and Their Eggs" informational inset, the picture at the bottom is captioned "Close relative THE SAND RATTLER". This implies that is an image of a Sand Rattler, however, that image is also used and labeled as a Venomous Vorpent in other official sources.
In any case, being close relatives to the Venomous Vorpent might imply that the Sand Rattler is very small, and has a long sinuous body as well.
There is no mention of the Sand Rattler's abilities. However, as a relative of the Venomous Vorpent, the Sand Rattler may also be poisonous to some degree. This is also implied in its name - "Rattler" evokes images of the poisonous Rattle snake.
The Sand Rattler is mentioned in an informational inset for the Venomous Vorpent in this book.