Evolution, but no evolution? | Pokemon Facts #90 - Shellder

 Shellder is a special pokemon, because it evolves, but it doesn not evolve. Basically, if you want a Slobro, you need a Shellder to bite Slowpoke's tail. and when this happens, Shellder "evolves". But it seems this evolution is not seen as an evolution.

• In the anime, Shellder is required to evolve Slowpoke into Slowbro or Slowking. Shellder clamps onto a Slowpoke's tail to evolve into Slowbro, or onto its head while wearing the King's Rock  to evolve into a Slowking. Even though Shellder changes form  drastically when it bonds with Slowpoke to evolve, it is not  considered an evolution in and of itself. This is pretty strange...And to be honest, it would be amazing to have a branched evolution, with the green form Shellder takes to evolve Slowpoke as a second evolution.

• In the episode The Evolution Solution, it is revealed that Shellder is still capable of speech as the shell of a Slowbro.

• Shellder were shown to be capable of floating through the air in Pokémon Snap, a trait that has been seen in no other media.

• Shellder and Cloyster share their category name with Clamperl. They are all known as the Bivalve Pokémon.

Thanks, Bulbapedia!