Slowpoke/Slowbro/Slowknig are probably the deepest and most thought out Pokemon of them all.
"Slowpoke tails drip an odd syrup-like fluid which many fish are attracted to. Slowpoke's evolution is based on the Pokémon, Shellder. If a Shellder clamps onto a slowpoke's tail, it will evolve into Slowbro."
Simple enough to remember, right?
"The Shellder which bites on its tail sucks on the aforementioned syrup, and so it is hard to remove. Also toxins which leak from the bite pass into Slowbro, which numbs its immunity to pain even more. The relationship between Slowpoke and Shellder is symbiotic; being attached to the Slowpoke enables the Shellder to travel on land, whereas the weight of the Shellder on its tail enables the Slowpoke to walk on its hind legs, freeing the use of its arms."
Starts to make sense. Until your mind is blown by Slowking.
"The Shellder biting Slowking's head injects toxins inside its brain every time Slowking yawns. The poisons interact with Slowking's neural tissue to unlock mental faculties hitherto untapped, making Slowking considerably smarter and more alert than the sluggish Slowpoke or Slowbro. This makes it stand out among the aforementioned species, which recognize it as a leader of sorts.
The anime adds a ritualistic test to select the leader of the Slowpoke and Slowbro. A group of Slowpoke gather in a Shellder-infested lake, at the middle of which laid a King's Rock. The Slowpoke that manages to dodge the Shellder and avoid evolving into Slowbro make it to the King's Rock and places it on its head, whereupon a Shellder clamps on the rock, triggering Slowpoke's evolution into Slowking."