Pikaberry
One logical explanation would be that slowpokes tail acts as a megastone of sorts for shellder, megaevolving him into a defensive measure for slowbro (higher def sp def an hp stats) since i cant recall ever seeing or hearing of spiral shellder in the wild by itself. This supports the shellder reverting back to ita original form if it got seperated. Thats just my theory
Edit: slowking has spiral shellder on his head so maybe slowpoke in general is like a mega stone to shellder
2nd edit: depending on how the shellder is removed, either by prying it off or amputating the tail still in his mouth then maybe a spiral shellder in the wild would be possible
This makes sense to me.
And I would assume that once the cut off tail ran out of nutrients the Shellder would spit it out, Effectively DeMega-Evolving.
New Item for Gen 7: Slowpoketailite.