You've got my interest on this. Since this is going to be a sidescroller, how will the monster see the player? Will the monster jump out from the background as the player passes? Anyhow, it's nicely drawn. And some aa would be good as Frous mentioned, for softening the areas where the darker colors meet the lighter if you deem it fiting.
You should add some anti-aliasing for making the sprite even smoother! 
No need for any AA, this is perfect. 10/10
Love the colours, love the design, love everything! Can't wait to see more!