Dragon Scale - Evolution Item
A scale that can make certain species of Pokémon evolve. It is very tough and inflexible.
Special evolution items like the Dragon Scale first arrived back with the second generation of Pokémon, and are now required when evolving Pokémon such as Kingdra. When evolving these Pokémon, this item is required in addition to the candy, and is consumed when used.
How to evolve Seadra into Kingdra
On your Seadra pokémon's screen, select the evolution through the usual in-game menu. The Dragon Scale and candy will be consumed, and you'll have a new Kingdra with hopefully all the right moves.
In Pokémon GO, evolving Seadra into Kingdra is a lot simpler than some of the other Pokémon games which required your Pokémon to also be traded to another player for the evolution to take place.
How to get the Dragon Scale
The Dragon Scale is obtained from spinning Pokestops and Gyms. There is a very low drop chance of around 1% per spin to receive a Dragon Scale or any of the other evolution item, and there is an equal chance of obtaining each of the 5 currently available items.
There's no way to ensure that the Dragon Scale will drop, but with only 4 other items, there's a greater than 50% chance that you'll receive one once you've spun over 250 Pokéstops.
The 7th day streak bonus gives you a very high chance of obtaining one of the evolution item, but it's not believed to be guaranteed.
Other evolution items
As of generation 3, the other evolution items that can be found in the game are: