Kingdra, the feared Pokemon of Clair & Juan
Kingdra is quite the popular and memorable Pokemon due to how tough it's defenses and offenses are late in the game that I'm actually surprised that Kingdra hasn't had any major appearances in the anime since Clair from Gen 2, like literally not even Gen 5 where Clair makes a reappearance and Gen 3 where Kingdra is the signature Pokemon of Juan. Really weird but outside of Clair & Juan, there's another Kingdra user that Pokemon Adventure fans know about and that's none other than Silver who you may know from the games as Gen 2's rival character. What's interesting about Silver's Kingdra is that it was originally a Horsea given to him by a very close friend and has since then grown up to become one of Silver's most powerful Pokemon.
Well-Known Trainers who caught or befriended the Kingdra evolution line.
Kingdra - Clair (Games, Adventures manga, Anime)
Kingdra - Juan (Games, Adventures manga)
Seadra - Lance (Games)
Kingdra - Silver (Adventures manga)
Horsea (Given) - Green/Blue (Fem Protag, Adventures Manga)
Horsea - Misty (Anime)
Kingdra (Imaginary) - Molly (Movie)
Seadra - Fergus (Movie)
Seadra - Cissy (Anime)
Swift Swim Kingdra
Kingdra
Item: Leftovers/Damp Rock/Assault Vest
Ability: Swift Swim
Nature: Modest Nature (+ Sp. Atk, - Atk)
EVs: 252 Sp. Atk/252 HP
Rain Dance/Hidden Power Poison (Assault Vest)/Hidden Power Grass (Assault Vest)
Hydro Pump/Scald
Draco Meteor/Dragon Pulse
Ice Beam
Kingdra is quite the monster on rain-dance teams. Even with the introducing of fairy-types and the much better bulkier hard hitting Mega Swampert, Kingdra's water-dragon typing bulk with decent offenses and the ability Swift Swim is still something to be feared. In fact, with no investment in it's speed with a non-speed nature, Kingdra can still outspeed almost every non-legendary unboosted Pokemon under Rain Dance that some players might think it's better to invest it in Kingdra's bulk and even further with Assault Vest if you have another Pokemon that activated Rain Dance.
Juan's Dragon Dance Kingdra
Kingdra
Item: Leftovers
Ability: Swift Swim or Sniper
Nature: Adamant Nature (+ Atk, - Sp. Atk)
EVs: 252 Atk/252 HP or 252 Atk/252 Spe
Dragon Dance
Waterfall
Outrage
Iron Head
Juan is acknowledged by many fans as Wallace's mentor and is the reason why Wallace uses so many stylish battle tactics as the "Contest Master", a title first introduced in Pokemon Adventures long before it was adapted as main series canon in ORAS. So even though it never happened in the games, it makes a lot of sense that a "Stylish" Kingdra under Juan has dragon dance don't you think? For this set, Dragon Dance is key to making Adamant Kingdra work and one could risk using it twice for extra bulk investment but one is enough for those who want to play it save with speed investment.