Deadly Nightshades were mentioned once in the series, in A Hero's Guide to Deadly Dragons.
There is no appearance or description of a Deadly Nightshade Dragon.
Because Deadly Nightshades are mentioned along with poisonous dragons, as well as the fact that nightshade plants are poisonous, we might assume the Deadly Nightshade is poisonous, too.
- "Was it that they couldn't stand the color yellow? No, that was Deadly Nightshades . . ."
It is entirely possible the author meant Toxic Nightshades. However, Toxic Nightshades were mentioned a few pages previously, in that they are dealt with by blowing in their eyes. In fact, Toxic Nightshades are yellow in color, so being put off by their own coloration doesn't make sense. Similarly, Deadly Nadders are also mentioned, and they are dealt with by rubbing their tummies with the end of a nettle.
Deadly Nightshades were mentioned briefly in this book, when Hiccup recalls a weakness of this dragon.