Probably not Pokemon

[x0_000]

They're based on Oni, as well as the Japanese phrase "oni ni kanabo" - "an ogre with an iron club" - which means total overkill. (info from the Obakemono project page, to give proper credit).

I know a fair bit of oni lore but I'm still worried about making the Pokemon themselves too complex.

I think the arm clubs are developed enough in the 2nd form, the third form feels "just like the 2nd, except BIGGER!!!", which sounds like a cop out to me. You could try making hands grow out of it for the 3rd form, or some other twist. You could also make the clubs different shapes, like the current one is more of a round shape, while you could try a more polygonal shape like here. Another thing you could try is having the tail grow a club too. You can also vary the number of horns on each on, in addition to the shape as you're currently doing. The 1st form could have 1 or none (incidentally the horns are red instead of white), and 3rd one could have 3 or 4.

By the way, you ARE going to make a blue one, yes?