Pokémon Go recently introduced item evolutions in to the game which allow you to evolve Gen 1 Pokémon in to exciting new Gen 2 Pokémon.
Items can be found in-game by spinning Pokéstops either at random or on the seventh day of a spinning streak, and then sit within your items list underneath your pokéballs and other items.
While the game tells you what items each Pokémon needs to evolve when you view its stats, it doesn't do a great job of telling you what you can use your items on when you acquire them, which can make finding out what they are for rather time consuming.
This guide provides a quick look for each items' use so you no longer need to search to evolve.
With 100 Horsea candy and a Dragon Scale, Seadra can be evolved into Kingdra.
With 50 Slowpoke candy and a Kings Rock, Slowpoke can evolve in to Slowking.
With 100 Poliway candy and a Kings Rock, Poliwhirl can evolve in to a Politoed.
With 100 Oddish candy and a Sun Stone, Gloom can evolve in to a Bellossom.
With 50 Sunkern candy and a Sun Stone, Gen 2 Sunkern can evolve in to a Sunflora.
With 50 Onix candy and a Metal Coat, Onix can evolve in to a Steelix.
With 50 Scyther candy and a Metal Coat, Scyther can evolve in to a Scizor.
With 50 Porygon Candy and a Up-Grade, Porygon can evolve in to a Porygon2.
That's all the possible item evolutions for now, but be sure to check back for further item evolutions as later generations are added!