These sprites need a lot of work. First, the line art is poorly done. The lines are too rough and jagged which is not a common thing in organic beings. Try rounding out your lines to make them look more "realistic". Next, the shading. You need to first pick where the light source is. If these are done like regular pokemon sprites then the light comes from the top left. So, the lighter shades should be closer to the light source. Of course, this doesn't apply to the entire sprite because places such as the under side of the sprites head will be darker but you get the point. If you don't understand how to shade properly, look at other sprites. If that doesn't work, try making concept art before hand. The concepts aren't too creative, but if you improve their line art and shading then that's fine.