Slowking is a bipedal Pokémon with a Shellder latched onto its head. It is primarily pink, with a pale yellow muzzle and scaled belly. Its feet and hands each have a single clawed digit, while the hands also have a clawless thumb. Around its neck is a spiny ruff with alternating red and white stripes. The Shellder on its head has a spiral shape with two horns near the base. A red gem resides in the center of the Shellder. The Shellder's eyes are visible on the back of the Shellder.
Shellder's bite releases venom into Slowpoke's brain, causing a chemical chain-reaction that unlocks a superior intelligence and extraordinary power. More venom is injected into Slowking every time it yawns. The more venom the Shellder releases, the more intelligent Slowking will become. However, if the Shellder on its head comes off, it will forget everything it has learned and would revert itself back to Slowpoke. Slowking's intelligence can match that of a human and it constantly attempts to learn more. Its inspiration and intuition are also highly developed, and Slowking is able to stay calm in any situation and plan situations with effective results. This allows Slowking to solve anything despite nobody understanding it. In the anime, it can understand and speak the human language.
In Galar, Shellder's bite causes a different chemical reaction in the brain due to the Galarica spices that make up Galarian Slowpoke's diet, causing Galarian Slowking to take on a unique appearance. It has a slight color difference with purple spots on its tail and paws, and has a long black collar. The Shellder has grown more dominant, appearing completely different with a darker gray coloration and larger size, covering Slowking's eyes and nostrils. At the center is a purple spot with a green orb and two black eyes. The horns on top of shell are three round spikes with two more spikes on the middle of shell. The bottom parts are sharp with two purple shaped tubes from which poison may be secreted. The Shellder's teeth have also grown in size and are broad and flat instead of sharp; the same holds true for its wearer.
Shellder's intelligence is said to have increased to the point its psychic powers put other Psychic-type Pokémon to shame. Shellder can also control Slowking. Galarian Slowking have been observed gathering in groups to chant mysterious spells to use in battle or communicate with each other. It is unknown what Galarian Slowking is trying to say as the incantations have yet to be fully understood. Galarian Slowking can create strange potions from its internal toxins and spells.
Slowking also appeared in I Choose You!, which is set in an alternate continuity from the main series.
In A Crowning Achievement, a colony of Slowpoke was attempting to reach the King's Rock in order to evolve into one so that they could help save the colony from a drought. The Slowpoke were faced with many problems since the King's Rock was also surrounded by Shellder, and whenever they failed to reach it, they would evolve into a Slowbro instead. One Slowpoke nicknamed Arthur, which was owned by Alice Telesu, tried reaching the King's Rock and eventually succeeded. In the end, he left Alice in order to be the leader of the colony. Arthur reappeared in Larvitar's vision in Address Unown.
Slowking, the Royal Pokémon. A Water and Psychic type. If a Shellder bites a Slowpoke's tail, Slowpoke evolves into Slowbro. But if a Shellder bites a Slowpoke's head, the Slowpoke evolves into Slowking with the intelligence of a genius.
A hint of Slowking's existence first appeared in Bang the Drum, Slowbro, while Lorelei was musing on Slowbro's enhanced abilities after it had been bitten by Shellder. She remarked that Slowpoke would likely have further improved abilities if it was bitten on its head.
InThe Final Dimensional Duel I, Charon was seen in possession of a Slowking. It was used to provoke Giratina into attacking Pearl, and later was used to attack Pearl again when Diamond was supposedly killed. It was later abandoned toward the end of the arc.
A talking Slowking appears in The Truth About The Mysterious Whirl Islands!!. It met Gold and Chris when they got stranded on Whirl Islands. It initially thought they came to challenge Lugia, but they knew nothing about the Silver Wing. It asked them to leave but the trees were too thick to cut and build a raft. It then offered to give them its rubber boat if they defeated a recently gone violent Kamex that had ruled the island for a century. They manage to defeat it, wrecking Slowking's cave in the process. It later explains to them about Lugia and the Whirl Islands after they defeat Lugia.
In the TCG, Slowking's Southern Islands card is the only card depicting a Pokémon introduced after Generation I to have a square evolution box in Japan only. In the English version, the Southern Islands collection used the Neo-era designs, which, like all later card layouts, feature a circular evolution box.
Johtonian Slowking shares its category with Pyroar. They are both known as the Royal Pokémon.
Slowking's shell makes it similar to hermit crabs, though it is physically similar to giant otters, giant salamanders, and hippopotamuses. Its shell "crown" may also be a reference to the mythical sazae-oni, a magical sea demon of Japanese folklore distinguished by its spiny "sazae" or "turban shaped" snail shell. Additionally, it seems to be based on a king, as the growth around its neck is based on a king's ruff while its shell is similar to a crown.
By contrast, Galarian Slowking seems to draw inspiration from popular depictions of mythical sorcerers or warlocks: its crown has morphed into a sorcerer's pointed hat, its elongated ruff seems akin to a long dark robe, and this regional variation has a predilection for "chanting mysterious spells" in battle.
Slowking is a combination of slow and king.
Yadoking is a combination of 宿借り yadokari (hermit crab) and king. Alternatively, it may also derive from 宿主 yadonushi ([parasitic] host), based on its relationship with Shellder in its evolutions.