Fenrisúlfr, Hati & Sköll

According to the lore regarding Ragnarök there are two wolves, named Hati and Sköll, who chase the moon and sun.

Hati will catch and devour the moon, Sköll will catch and devour the sun and all the stars will cease to shine. The earth will shatter and shake, mountains will come crashing down and this will enable Fenrisúlfr to break free of his chains, signalling the beginning of Ragnarök.
Artwork by exileden.

Jörmungandr – The Midgard Serpent

In Norse mythology, the tides and waves of the ocean are caused by Jormungandr, a great serpent with its head latched onto its tail, turning continuously out in the ocean. Jormungandr was tossed into the sea by Odin and grew so large that he encircled the world.

On the day of Ragnarok, Thor and Jormungandr will do battle and kill fatally wound one another. 

(Reference 1) (Reference 2)

Notes:

The Midgard Serpent was requested by queerlittlemermaid. If you have any requests, feel free to drop them in my Ask Box or tweet them @WorldofMyth.