The first one is a huge step up from the other two. Good work on that one even though the colored outlining seems very sparatic and unplanned.

This is actually pretty cool but would definitely fit better in the dragon warrior monsters game ;D

Try making the shadow ball into a ball shape, unless you were going the hexagonish shape. Check out the anatomy of a real skeleton just so it looks plausible, but then again, it is a Pokemon and you're supposed to let your imagination run free. The skirt is unneeded and I think the fact that the first evolution has bigger feet and a bigger head make the third evolution not too intimidating.

well, everyone said, even in the brazilian pixel art forums, that the third is the worst =/

however, I don't know how to improve it...

what should I do?

reminds me of the weedle / caterpie family.

good concept, but i think the skeleton ones could have been done a bit better. (the baby one is much better though, IMO.)

there's not much contrast and the bones on the last one (along with what chris said) are a little too stright. good concept though

Arms are too long on the last one, and to big on the lower part.