Slowking are Slowpoke who were bitten on the head by a Shellder. This is often the result of a Slowpoke holding a King's Rock because a Slowpoke holding this item will attract Shellder to bite its head, rather than its tail. Shellder's bite begins to release venom into Slowpoke's brain, causing a chemical chain-reaction which unlocks a superior intelligence and extraordinary power. The more venom the Shellder releases, the more intelligent Slowking will become. However, if the Shellder on its head come off, it will forget everything it has learned.
In appearance, Slowking looks much like a bipedal Slowpoke with its Shellder resembling a crown, although the shape of Slowking's eyes and the scales on its belly cause it to differ from its pre-evolution. There is a small red gem on the upper part of the "crown." Slowking also have a spiny red and white ruff around its neck. Upon evolving, its tail becomes shorter and presumably no longer produces the sap-like substance that it did as Slowpoke. Much like Slowbro, Slowking has a scaled underbelly.
The Shellder on its head is apparently not as tightly clamped on as one would be on a Slowbro's tail. This is likely due to the absence of the sweet tasting sappy substance it would taste if attached to a Slowbro's tail. As it is possible for the Shellder to detach and fall off, but not cause a devolution, it's likely Slowking's evolution is permanent and irreversible for both Pokémon involved. However, as mentioned the Shellder must remain attached to Slowking for it to retain its intelligence and memory. Whether or not Slowking's mental state reverts to that of a Slowpoke is unknown.
Due to the way Shellder's poison reacted with its brain, Slowking possess an amazing intelligence, to the point that it can match that of a human's and can understand and speak the human language. They can plan situations with effective results. Every time Slowking yawns, Shellder injects more poison into it, making it even more intelligent.
Slowking can use Nasty Plot and the rarely seen Trump Card attack while being unable to learn Slack Off and Amnesia on its own (those two moves would have to come from the Slowpoke it evolved from).
Because Slowking is much more intelligent than Slowpoke or Slowbro, wild Slowking must lead colonies of wild Slowpoke and Slowbro in times of crisis. It has such an easy-going personality that it never gets stressed.
Slowking lives with the Slowpoke and Slowbro at their paradise to lead them.
It is very likely that Slowking is an omnivore. It may also feed on Fish as well, much like Slowpoke and Slowbro.
This Pokémon was unavailable prior to Generation I.