How to Get Kingdra in "SoulSilver"by Joshua Phillips ; Updated September 22, 2017
"Pokémon SoulSilver" is a remake of "Pokemon Silver" that features many of the original 150 Pokémon, such as Horsea, a Water-type Pokémon. In "SoulSilver," however, Horsea can evolve into a new form, previously unseen in the original "Pokémon Silver": Kingdra, a Water and Dragon-type Pokémon. The only way to get a Kingdra in "SoulSilver" is to evolve Horsea into Seadra and then trade Seadra while it holds a special item.
Catch a Horsea by fishing or surfing at the Seafoam Islands or Whirl Islands. You can encounter a Horsea at levels ranging from 10 to 40, depending on the location. Catch a Horsea by using any Pokémon move that affects status, such as Stun Spore or Sleep Powder. Once you have incapacitated Horsea, use a high level Poké Ball, such as an Ultra Ball, to capture him.
Obtain a Dragon Scale at the Pokéathlon Dome for 2,500 Pokéathlon Points. Alternatively, you can find the Dragon Scale on wild Pokémon, such as Horsea or Dratini, or at Mt. Mortar in a Poké Ball.
Battle wild Pokémon and other trainers with Horsea until he reaches level 32. Once Horsea reaches level 32, he will automatically evolve into Seadra.
Tap "Pkmn" on the bottom screen. Tap Seadra and then tap "Item." Tap "Give" and give Seadra the Dragon Scale to hold.
Trade Seadra to a friend at any Pokémon Center while Seadra is holding the Dragon Scale. Once the trade is complete, Seadra will automatically evolve into Kingdra. Have your friend trade the Pokémon back to you to receive your Kingdra.
These steps also apply to "Pokémon HeartGold."