Change is a normal part of Pokémon evolution, but the design differences between Shellder's usual form and the way it looks on Slowbro's tail has perplexed many fans since Pokémon Red and Blue. For a long time, fans asking how the two shelled creatures were the same Pokémon went unanswered, but the Pokémon universe now has an explanation.
For those unfamiliar: Yes, that toothy, spiky, spiraled shell on Slowbro's tail is supposed to be Shellder, the goofy purple clam Pokémon. There are plenty of odd logical leaps that make the Pokémon universe make no sense, but this strange fact is one of the series' most confusing. It almost seems like Game Freak created Slowbro's design without thinking about where the shell - clearly a separate creature from Slowbro, given its eyes and mouth - came from. Then, in the absence of any other shell-like Pokémon at the time, it seems Game Freak simply slapped Shellder's name into Slowbro's Pokédex entry.
Until recently, the Pokédex had only revealed a few facts about Slowbro: Rather than being an entirely new Pokémon, it is essentially just a Slowpoke with a Shellder parasite attached. It will revert back to Slowbro if the Shellder is somehow broken off, but the Shellder is unlikely to let go, since it enjoys the "tasty flavor" oozing from Slowbro's tail. Still, the Shellder's presence does appear to have changed Slowpoke's powers and anatomy enough for it to be classified as a new Pokémon. Venom secreted by the Shellder has made Slowbro immune to pain, and Slowbro will sometimes "become inspired" if the Sheller bites down harder. None of this explained Shellder's new look, though, confusing fans like Twitter mana mana.
In 2011, Pokémon fan Pokémon Ultra Sun and Ultra Moon Pokédex entries:
"Shellder, in its greed to suck out more and more sweetness from Slowbro's tail, has metamorphosed into a spiral-shaped shell."
It's unclear if Slowbro's sweet liquid was what changed Shellder or if it was the Pokémon's greed itself that twisted its form, but this at least provides some explanation (however vague) for Shellder's odd appearance.
Since Slowbro's introduction, a few more Pokémon pairs have influenced one another's evolutions. The Pokémon Sword and Shield's Expansion DLC suggests Shellder bit Slowpoke's head instead of its tail, imbuing it with more poison and possibly taking control entirely, so perhaps Game Freak will reveal more about the relationship between these two Pokémon soon.
