I wonder why the Shellder on Slowbro's tail and on Slowking's head looks different to the Shellder we normally see.
It probably evolves/transforms along with the Pokémon to be able to clamp onto the tail/head in a form that comes in most handy. I guess it would be hard for a Shellder to clamb to a crown and a Slowpoke head or a tail in it's normal form.