Prepare for a bitching:
Firstly, Cubone seems a bit too chubby, that may or may not be a style choice but Cubone was never chubby, just round.
Secondly, the color scheme is off, Cubone doesn't have a white belly, it's a lighter brown of his regular colors.
He also doesn't have the belly color going up to his mouth, take a look here:
http://www.kidslearnfromplay.com/wordmat...Cubone.png
Next, the bone is oddly shaded, he has good shading and white lighting on his skull but none on his bone? Kinda confuses the lightsource, and just because its behind/beside him doesn't mean for it to be excessively dark. IF anything:
Copy the light bone.
Add white on the light bone.
Then take the copy of the original and replace it with the dark bone, it would make a but more sense shading wise.
His frontmost leg also seems to be facing too much to the right, and Cubone is only looking halfway of that direction.
As everyone already pointed out, the animation seems like he's taking baby steps, now I know it's tough considering there's no ref of that, but if he's gonna walk, he's gonna need a lot more movement and framework, meaning his skull will have to bob a bit, and his body will have to shift and not face the same direction the whole time, and his legs will have to move a bit more. But it's up to you if you want to do all that, if anything you could make it comical and have him walk funny on purpose, but that's just a suggestion.
Anyway it's a good sprite, and nice choice of pokemon, and I'm the biggest asshole on the planet when it comes to critting pokemon sprites, it may be a lot, but it'll help point out what to fix.