The Poisonous Puffleworm appears in A Hero's Guide To Deadly Dragons. They are extremely poisonous dragons, but they can be immobilised by whistling. Hiccup uses this trick to crawl through a secret tunnel filled with Poisonous Piffleworms to escpe from the Meathead Library. It says in his book he saw a Lesser-spotted Squirrel Serpent whistle to immobilise Poisonous Piffleworms on Wild Dragon Cliff.
The Poisonous Piffleworm was also hiding in Hiccup Horrendous Haddock II's Hero's Guide to Deadly Dragons. This was found out when Hiccup opened the book, as he temporarily forgot how to immobilise the Poisonous Pif