While I cant be sure what hte artist intended the 'dragon' to be, I would assume - keeping with norse mythology - that Thor is fighting Jormungandr, the world serpent, in this image. Jormungandr was said to encircle the realm of Midgard, the mortal world, in norse legend.

dat ant' no dragon

You're gonna need a bigger hammer.

I wonder how they would ever manage to fit Jormungandr in, if they wanted to incorporate him in the movies with Thor and the Avengers.

Hydrogen said:
You're gonna need a bigger hammer.

Nope, Mjölnir, is the strongest warhammer in all of the world(or that's waht ol Norse mythology says) controls the weather and is indestructable!

This is great.

Thor's mind: "And in case you still haven't noticed it: I! AM! EPIC!!"

Nice artwork

hydrogen said:
You're gonna need a bigger hammer.

Fun fact, Mjolnir has the power to increase it's size.

Also, Top ten anime fights where the main character dies, or top ten pictures taken moments before disaster.

Someone add the Jörmungandr (Midgårdsormen) tag.