Cepheus

That I must disagree entirely with. Cradily is based off a Sea lily. It should not even be a Grass type since sea lilies are sedentary animals, not plants, and do not photosynthesize. Bellsprout's line, however, is clearly a Nepenthes, or pitcher plant. No relation at all.

However this is Pokémon, so feel free to theorize. Castform could be an ancestor to Thundurus for all you know.

I think Castform is manmade pokemon that was created to predict the weather. What if they made Castform from Thundurus and Tornadus clouds, but not from Landorus so that explains why it doesn;t have a sand form?

I was attempting to make a dry joke, but looks like it failed.

Anyways, Castform is just ass. I like how it is supposed to predict the weather with 'Forecast' as an ability, but it can only respond to existing weather. Not really doing any prediction.