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.
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. 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. In the anime, it can understand and speak the human language. According to The Complete Pokémon Pocket Guide, it has such an easy-going personality that it is never stressed. Slowking is rarely found in the wild, though it can be found by the water's edge. It has been seen eating seaweed and drinking coconut milk. In Alola, Slowking occasionally matches wits with Oranguru by the beach.
Slowking reappeared in an alternate reality of the main series in I Choose You!.
Slowking made its main series debut in A Crowning Achievement, where a colony of Slowpoke were 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. It reappeared in Larvitar and Unown's vision in Address Unown.
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 "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 Slowkig'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 English, the Southern Islands collection used the Neo-era designs, which, like all later card layouts, feature a circular evolution box.
Slowking shares its category name 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. It is also 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.
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.