Odd One Out (minus the double posts!)

[Sirfetch’d]

Magneton is the only one that can use Eviolite!

Onix, Seviper, Arbok

 

[Lycanthropy]

Onix is no poison type Pokémon.

Electrode, Klinklang, Magnezone

 

[Zoroark Cutie]

Onix is the only one who is a Rock/Ground type.
Sniped'd
Magnezone is the only one who was introduced in generations later than its first stage.
Quagsire, Whishcash, and Gastrodon

 

[Sirfetch’d]

Whiscash learns Dragon Dance while the others do not!

Salamence, Dragonite, Garchomp

 

[blue]

Garchomp is the only one that isn't part Flying-type!

Huntail, Miltank, Bouffalant.

 

[Lycanthropy]

Huntail is the only one not based on a cow (or similar).

Tynamo, Ralts, Nidoran♂

 

[Sirfetch’d]

Ralts is the only one with a mega evolution in the family.

Quagsire, Clefable, Seaking

 

[Zoroark Cutie]

Seaking is the only one without an immunity.

Ekans, Vulpix, and Psyduck.

 

[Charlie Brown]

Ekans is the only one not used by one of Ash's travelling partners in the Kanto anime.

Dratini, Bagon, Gible.

 

[kingdenas]

Gible is the only one that evolves into a part ground-type!

Floatzel, Starmie, Lucario

 

[Charlie Brown]

Starmie is the only one that isn't a Gym Leader's ace Pokemon in Sinnoh!

Dewott, Wartortle, Croconaw.

 

[Ace Trainer 188]

Dewott can't learn Bite via level up. But I wished it could.

Keldeo is an event Legendary Pokemon

You got the right mon... But the wrong explanation. Keldeo ain't a cover mascot.

You guys seem to keep guessing wrong lately. Lets do a super easy one:
Jynx, Electabuzz, Magmar .

 

[DyingWillFlareon]

Jynx has no third evo?

Slugma, Sandshrew, Venonat

 

[Corrin]

Venonat is the only one weak to fire moves.

Gible, Bagon, Axew

 

[Ace Trainer 188]

Axew's final evo isn't dual typed!

Gengar, Charizard, Alakazam!

 

[Corrin]

Charizard can't learn Shadow Ball

Pidgeot, Braviary, Staraptor

 

[Judge Mandolore Shepard]

Braviary since it is not the final evolution of an early game Flying type Pokemon

Machamp, Alakazam, Poliwrath

 

[Zoroark Cutie]

Poliwrath is the only that you can get through evolution with a stone, the other 2 you get through trade.

Spinda, Zangoose, and Kecleon

 

[Corrin]

Zangoose is version exclusive

Pumpkaboo, Phantump, Ghastly

 

[Otamajakushi]

The first two are Ghost/Grass, and Ghastly is Ghost/Poison

Nincada, Pidove, Skarmory