Shellder

Shellder is a clam-like Pokemon. It is a pure Water type. Shellder has some unique evolutionary properties. It can only evolve if someone uses a Waterstone on it. Shellder also plays a few major roles in Slowpoke's evolution. If it attaches itself to Slowpoke's tail, it can evolve into a Slowbro with Shellder taking on the appearance of the large, spiky shell that engulfs and stays attached to its tail. If it attaches itself to Slowpoke's head, then it can evolve into a Slowking, in which it secretes venom into its brain, thereby unlocking its superior intelligence.