Monstrous Strangulator

The Monstrous Strangulator is an octopus-like dragon that appeared in the book How to Be a Pirate. It first appears when Hiccup and Alvin the Treacherous were fighting in an underwater cave that was full of treasure. When Alvin stepped on a pile of treasure during the fight, the Monstrous Strangulator reached up from underneath the treasure, grabbed Alvin, and ate him. Hiccup escapes the Monstrous Strangulator by tricking it into poisoning itself.