The Hairy Scary Librarian is the main antagonist of the How to Train Your Dragon book 6, A Hero's Guide to Deadly Dragons.
How to Train Your Dragon
Gobber the Belch mentioned that he stole the book, How to Train Your Dragon from Hairy Scary Librarian from the Meathead Library, because that book is very essential for drahon trainers, but that book has only few sentences, all of these are "The golden rule of dragon training is.... YELL AT IT! (The louder the better.)" as its only chapter.
A Hero's Guide to Deadly Dragons
After Toothless burned the How to Train Your Dragon book, Hiccup, Fishlegs and Camicazi go in the library to steal another book of the same title. They met Hairy Scary Librarian and he is about to kill Hiccup and Camicazi, but Fishlegs used a book to slap him to save them. They stole the book they are looking for. Hiccup saw a book, which was written by Hiccup Horrendous haddock II, who is his great uncle. He got it and saw a passage and escaped.
Back on Berk, Hairy Scary Librarian is about to kill Stoick, but the librarian got smashed by the dragon Hiccup and his friends are riding, but the librarian is still alive. He later got kidnapped by Madguts the Murderous treated him as a slave.
He appeared as a tall old man with very long beard. He also has has two swords named the Heart-Slicers. It is also stated that he has size 18 shoes.
He is a very territorial type. He does not allow anybody to enter his closed library and he is very homicidal and wrathful to any intruder.