It's a widely known fact among Pokémon Trainers that Shellder clamps onto Slowpoke's tail and becomes a Slowbro. What doesn't make sense, however, is the shape that Shellder suddenly takes. To explain this away, maybe an Alolan Shellder is the form that the clam-like Pokémon takes when it bites onto Slowpoke's tail?
An explanation as to why Shellder takes on this form in Alola could be that the waters in certain areas of Alola give off the same chemical that Shellders are looking for when they bite Slowpokes. This would possibly change Shellder into a Psychic/Ice type (reflecting the gain of Psychic typing originally), or possibly a new typing altogether.