Stealth Dragon

Stealth Dragons are undescribable because they are like chameleons, changing their appeareance wherever they are.

In the books
In A hero's guide to Deadly Dragons, a stealth dragon is stolen by Big-Boobied Bertha from Madguts the Murderous. Hiccup and Camicazi rode on it from the Bog-Burglars' Dragon Stables to the Forbidden Library to get another copy to get another copy of How to Train Your Dragon by Professor Yobbish because the original copy was ruined by Toothless.