There were many dangers in Norse mythology, but none were as deadly as the monsters that roamed the nine worlds:
Fafnir was once a dwarf but was twisted by Andvari's riches and became a dragon whose only purpose was to guard his horde of treasure. He moved without a sound, was deadly cunning, and breathed toxic poison.
To hear how Fafnir came to be, click here.
'Fafnir guarding the gold hoard' by Arthur Rackham.
Fenrir was a great wolf. He was bound by the Aesir because of the destruction he might bring to the nine worlds if he was allowed to roam freely. It was prophesied, however, that at Ragnarok, Fenrir would break his bonds, kill Odin, and devour the world.
'Odin and Fenris' by Dorothy Hardy
Jormungand was known as the Midgard Serpent. It was an enormous, black-scaled snake that lived in the oceans surrounding Midgard.
'Thor and the Midgard Serpent' by Emil Doepler
Garm was a dog with jet-black fur and fangs stained with dry blood. To those that saw him, it was easy to mistake him for a wolf, but Garm was no wolf. He was much, much worse.
Nidhogg was a serpent that slivered on the bark of Yggdrasil. It leeched the tree's nutrients and gnawed at its roots.
Skoll and Hati were wolves hellbent on chasing down and devouring the moon and the sun. They were relentless, and they never tired.
'The Wolves Pursuing Sól and Máni' by J. C. Dollman