Slowbro is a mistery...It evolves with the help of Shellder, at least in the anime. But we don't know why this happens. And we don't even have an official form for the green Pokemon which Shellder transforms into when it bites Slowpoke. Below we have some facts taken from Bulbapedia.
- Despite it being a commonly known and accepted fact that Shellder triggers its evolution, mentioned in most of its Pokédex entries, Slowbro's evolution from Slowpoke has nothing to do with Shellder in the core series, unlike how the evolution of Mantyke requires a Remoraid.
- Mega Slowbro is tied with Cloyster for the highest base Defense stat of all Water-type Pokémon.
- Mega Slowbro has the highest base Defense stat of all Psychic-type Pokémon.
- Slowbro's Pokédex entries state that if the Shellder is removed, it will revert to its Slowpoke form.
- Many Pokémon are noted to evolve by two or more Pokémon combining, such as Metang being formed by two Beldum. However, the anime does not show this and simply spawns the second Pokémon out of nowhere: an example is Morrison's Beldum, which simply evolved into Metang without the need of joining to another Beldum. The evolution of Slowpoke into Slowbro or Slowking is the only time that evolution by two Pokémon physically merging is shown.