Timberjack

The Timberjack is the name of a gigantic species of dragon. It appears to have a head that is of a similar shape to the Monstrous Nightmare  and has a long slender neck, body, and tail. Its wingspan is huge such as the typhoomerang and each wing has at least 2 hooks, indicating that it has four "limbs" total coming out of its sides. It is confirmed in the Dragon Training Academy game on the HTTYD website that it walks using these limbs to move around, like "legs."

How To Train Your Dragon (film)
The Timberjack's only appearance in the film is in the Dragon Manual when Hiccup is attempting to research information on Night Furies. Like most dragons in the Dragon Manual, the Timberjack is considered extremely dangerous and should be killed on sight.

In Book of Dragons
In Book of Dragons, it is described in the manual as being one of the dragons in the Sharp Class, which are marked by vanity and a sharp weapon (in this case, the Timberjack's wings). This dragon attacks from out of the sky with its razor-sharp wings. It can slice through a forest of trees like a giant guillotine. It also has the ability to shoot hot, explosive embers. The Timberjack possesses no arms or legs, only its wings and long tail; but has been seen to stand up on its tail like a snake. Unfortunately, its long wings make it impossible for the dragon to scratch its back. However, it has been discovered that if one scratches any hard-to-reach places for it, the Timberjack will be your friend forever.

Trivia

 * Timberjack one of the bigger dragons like the Scauldron, Typhoomerang, Boneknapper, Whispering Death, Screaming Death and the Snaptrapper.
 * The Typhoomerang's body shape design may have been based on a Timberjack.
 * Given the above note (if it is true), the Timberjack might not appear in Wild Skies, as it would basically be the same as having another Typhoomerang.
 * The Timberjack's name is a combination of the words timber and lumberjack.
 * The Timberjack is the only Sharp class dragon without any poison. Both the Deadly Nadder and the Speed Stinger have paralyzing venom on their tails.
 * In School of Dragons, there is a Timberjack named Stokehead. It is unknown if he will appear in the tv series.