Seadra evolves into Kingdra when the player trades it while holding the Dragon Scale. The Dragon Scale item was introduced in Generation II along with Kingdra.
Seadra is actually the evolved form of Horsea when it reaches level 32. As a water-type Pokemon, Seadra resembles a seahorse with a long snout. Its head, sides and back are covered with spikes, while its torso is covered with cream-colored scales. It can poison its enemy using its spikes and pectoral fins with the Poison Point special ability, and it is the only non-poison type Pokemon that can use this move.
Evolving Seadra requires the Dragon Scale, which is a type of item that cannot be bought. The player can look for this item at Mt. Mortar in the Johto region games and at the Pokeathlon Dome in the HeartGold and SoulSilver game versions.