Awesome

This is the style of pixel art I really love.

I like this!

I like this very much, but there's a few things keeping me from giving it a full 10/10

First, and the simplest thing is the shade. From the looks of it, the light comes from somewhere up, back and to the right. Even so, the monsters right legs have highlights on them all the way down when the crab body should have cast a shadow on them.
Also the legs are a bit complicated and looks a bit messy as it is, and it takes me a few seconds to see what is the leg, what is the background, and what is the other leg. But, this might change automatically when/if it's animated on a background that hasn't this much of a similar color compared to the legs.

Now here comes the hard part: I noticed the canon on the crab's body, and at first, I thought it was supposed to be pointing straight upwards. Well, it took me a moment to realize that it was in fact pointing slightly forward as a canon.
now, I don't know how to fix that, except for changing the basic design, using two canons instead of one. That way, it would look natural if they were slightly rotated, meaning it would be easier to understand that it was pointing forwards.

Hope that helped, have a nice day!