Firstly, I'm extremely harsh when I critique things, don't take anything I say too badly as overall it's a good drawing.
Firstly, the lines are a little thick and, well, they look a little fudged at times. There's nothing too bad about them, they just aren't as neat as they could be. Also, the proportions aren't perfect (although they're good).
The colour is a little strange. It's not quite right if you were going for the light hitting it in a certain place, and it's far from a solid colour as well, as would generally be expected of a metapod.
The origionality is great. Who'd have thought of doing a metapod? I'd certainly never have thought to do one, you generally only see the first or last of an evolution chain.
Overall, it's a good idea, but it seems a little inexpert. It's a good start though, I'll have a look at any other work you do to see how you're doing.