I have this theory that Pokemon are basically products of their surroundings.
I know this sounds kind of obvious, but I noticed that Gens set further back in time tend to have Pokemon that look more 'natural' than the newer ones.
I think Sawk and Throh are relatively newly discovered Pokemon that are new (Headcanon I know).
I dunno, makes more sense in my head. Trubbish is a newer Pokemon, along with most Gen V Pokes. I think Magnemite is relatively new in proportion to Gen I.
I think in X/Y we'll see a lot more old designs, like Pokemon that are less based around man-made things.