As the Pokémon series has progressed, the ways in which the creatures can evolve has increased dramatically. There are some Pokémon that had to wait a couple of generations before they gained new evolutions.
Slowpoke > Slowking
Slowpoke decided that the thing clasped onto its tail would make for a fancy hat. All you need to do is trade it while it holds a King’s Rock.
The Eeveelutions
Eevee has received the most evolutions out of any creature in the series. The friendship & day and night cycle in Pokémon Gold & Silver brought Espeon and Umbreon to the series. You need a Moss Rock and an Ice Rock to create Leafeon and Glaceon in later games. Teach Eevee a Fairy-type move and show it affection to create Sylveon.
Chansey > Blissey
The one true powerhouse of Red & Blue became even stronger in Gold & Silver. You need to be really friendly to Chansey in order to make it evolve.
Yanma > Yanmega
Yanma was a Bug/Flying-type Pokémon that was introduced in Gold & Silver. It can evolve when it levels up while knowing Ancient Power.
Togetic > Togekiss
Misty’s beloved baby Pokémon gained a new form in Diamond & Pearl. You need to give a Shiny Stone to a Togetic to create Togekiss.
Aipom > Ambipom
Aipom gains a new tail when it evolves into Ambipom. This makes sense, as it needs to know Double Hit in order to be able to evolve.
Kirlia > Gallade
The famous waifu Pokémon was traded for a husbandu in Diamond & Pearl. The male version of Kirlia gained an evolution, which required the use of a Dawn Stone.
Magneton > Magnezone
It seems like Magnemite just keeps on changing. It became an Electric/Steel-type Pokémon in Gold & Silver. Magnemite later gained a new final evolution in Diamond & Pearl, which requires the use of a special magnetic field.
Scyther > Scizor
Scyther gained a new evolution in Pokémon Gold & Silver, which required it to be traded while holding a Metal Coat. Scizor will soon be battling enemies in the upcoming Pokkén Tournament DX for the Nintendo Switch.
Nosepass > Probopass
The Easter Island Pokémon gained a powerful new Grouch Marx form in Diamond & Pearl. It needs to be leveled up within a special magnetic field.
Porygon2 > Porygon-Z
After Porygon defeated Piccolo and married Chi-Chi, it became Porygon-Z. The other way to get one is to trade Porygon2 while it is holding a Dubious Disc.
Porygon > Porygon2
The Pokémon who shall not be named in the anime gained a new evolution in Gold & Silver. It needs to be traded while holding an Up-Grade.
Sneasal > Weavile
Sneasal gained a new evolution in Pokémon Diamond & Pearl. It needs to be holding a Razor Claw while it is being leveled up at night.
Rhydon > Rhyperior
The first true Pokémon in the series gained a new evolution in Pokémon Diamond & Pearl. It needs to be traded while holding a Protector.
Tangela > Tangrowth
This Pokémon goes to college and gets really into reggae in Pokémon Diamond & Pearl. You need to level up a Tangela while it knows Ancient Power to turn it into Tangrowth.
Gloom > Bellossom
The Oddish line doesn’t always have to end with a mushroom based on one of the most disgusting fungi in real life. It can turn into a happy flower in Gold & Silver when it is exposed to a Sun Stone.
Golbat > Crobat
It turns out that Red was just a jerk all along. If you want to evolve a Golbat, then you need to improve its friendship stat in Gold & Silver.
Dusclops > Dusknoir
A Dusclops from Hoenn can evolve into a Dusknoir in Sinnoh. It needs to be traded while holding a Reaper Cloth.
Electabuzz > Electivire
Electabuzz had to wait until Pokémon Diamond & Pearl to gain a new evolution. It needs to be traded while holding an Electirizer.
Snorunt > Froslass
A female Snorunt can be evolved in Pokémon Diamond & Pearl. It requires the use of a Dawn Stone. This new Froslass can then begin freezing corpses and displaying them in its home (check the Pokédex if you don’t believe us).
Gligar > Gliscor
A Gligar can evolve into a Gliscor in Pokémon Diamond & Pearl. It needs to be leveled up at night, while it is holding a Razor Fang.
Murkrow > Honchkrow
Pokémon Diamond & Pearl gave Murkrow an evolution which grew the hair on its chest. It needs a Dusk Stone (or a whole lot of Rogaine) to get there.
Seadra > Kingdra
You might think it takes a King’s Rock to turn Seadra into a Kingdra, but you’d be wrong. You actually need to trade it while it holds a Dragon Scale.
Lickitung > Lickilicky
One of Jessie’s old Pokémon gained a new evolution in Diamond & Pearl. You need to level up a Lickitung while it knows Rollout in order to make a Lickilicky.
Magmar > Magmortar
One of the best Fire-type Pokémon ever gained a new evolution in Diamond & Pearl. You need to trade it while it holds a Magmarizer.
Piloswine > Mamoswine
The wooly mammoth Pokémon gained a new evolution in Diamond & Pearl. You need to level up a Piloswine that knows Ancient Power to make a Mamoswine.
Misdreavus > Mismagius
This mystical Pokémon was introduced in Diamond & Pearl. You need to expose a Misdreavus to a Dusk Stone in order to make a Mismagius.
Poliwhirl > Politoed
It makes sense that the tadpole Pokémon should have a toad form. You could make this Pokémon in Gold & Silver, by trading a Poliwhirl that is holding a King’s Rock. This will make a Politoed.
Onix > Steelix
Brock’s signature Pokémon became a far more powerful threat in Gold & Silver. You need to trade an Onix that is holding a Metal Coat to make a Steelix.
