Odd one out!

[metroid711]

Mewtwo is the only legendary

Ferrothorn, Venusaur, and Vileplume

 

[KNS27]

Ferrothron only Pokemon Who get Grass/Steel type.(Also Ferrothorn introduced in Gen 5 when the rest of them in Gen 1)

Marowak, Muk, Electabuzz

 

[FeraligatrMaster]

Marowak has never been a Raid Boss in Pokemon Go.

Pyukumuku, Togedemaru, and Jangmo-o

 

[KNS27]

Jangmo-o only Pokemon that Evolution

Vulpix, Xerneas, Kyurem

 

[SirDolan]

Vulpix isn't a legendary.

Mega Charizard-X, Mega Sceptile, Mega Mega Swampert.

 

[Absolitetion]

Mega Swampert ain't a Dragon-type.

Zoroark, Ninetales, Fennekin.

 

[Ace Trainer 188]

Fennekin isnt fully evolved

Gourgeist, Dhelmise, Decidueye

 

[FeraligatrMaster]

Gourgeist is from Gen 6.

Haxorus, Druddigon, and Hydreigon

 

[Ace Trainer 188]

Druddigon doesn't have pre evos.

Electrode, Sandslash, Victreebel

 

[KNS27]

Victreebel has the dual type.

Piplup, Popplio, Froakie

 

[CF1994]

Froakie is not part of the Field Egg Group.

Dragonite, Noivern, Altaria

 

[VisionofMilotic]

This has a more than one answer, but let us say Dragonite because he is the only three stage evolution.

Dragonite, Garchomp, Hydreigon
This one is kinda subtle.

 

[KNS27]

only Hydreigon 4X Weakness with Fairy while Dragonite and Garchomp 4x Weak to Ice.

Fennekin, Chimcar, Litten

 

[FeraligatrMaster]

Fennekin's final evolution has nothing to do with martial arts.

Bulbasaur, Chikorita, and Turtwig

 

[KNS27]

Cikorita final Evo only have one type

Pichu, Pikachu, Raichu

 

[VisionofMilotic]

Pichu! Its the only one that can't breed, its a baby Pokemon.

Marowak, Hypno and Primeape.

 

[Ace Trainer 188]

Hypno isnt a physical attacker.

Chandelure, Cradily, Gardevoir

 

[KNS27]

Cradily only evolution once.

Gengar, Gyarados, Gigalith.

 

[VisionofMilotic]

Magikarp evolves into Gyarados by level up, not by trade.

Ninetales, Cloyster, Raichu

 

[KNS27]

Cloyster doesn't get Alolan Forms

Wailord, Golduck, Lumineon