Odd one out!

[Ida13]

Tentacool lives in water.
Articuno, Rockruff, Umbreon.

 

[FeraligatrMaster]

Rockruff isn't made to be defensive.

Throh, Haxorus, and Excadrill

 

[PageEmp]

Throh can't have Mold breaker as an ability.

Druddigon, Landorus, Ferrothorn

 

[SirDolan]

Ferrothorn can't have Sheer Force as an ability.

Kabutops, Armaldo, Tyrantrum

 

[PageEmp]

Armaldo isn't weak to fighting.

Flygon, Alakazam, Pidgeot

 

[FeraligatrMaster]

Alakazam can't fly.

Araquanid, Palossand, and Shiinotic

 

[KNS27]

Palossand and their Evolution has Type Matchup that not introduced in previous Generation.

Garbodor, Muk, Weezing.

 

[metroid711]

Muk, it's the only one to get an alolan form?

Vikavolt, Butterfree, Beedrill

 

[KNS27]

Pretty sure Vikavolt Only Pokemon in the list that evolution at the certain location.

Dewgong, Glaceon, Mamoswine.

 

[metroid711]

Glaceon because its mono-ice while the other two are dual ice

Muk, Gastly, and Lickitung
(this is kinda hard i guess)

 

[SirDolan]

Lickitung as it's a normal type while Muk and Gastly are both Poison types.

Mareep, Electrike and Shinx.

 

[metroid711]

Electrike evolves at level 26 while the other two evolve at level 15

Steelix, Metagross, Empoleon

 

[faf]

Empoleon doesn't have a Mega Evolution.
Scrafty, Heliolisk, Salazzle

 

[metroid711]

Scrafty, its the only one not in the monster egg group that the other two are in.

Turtonator, Salamence, Garchomp

 

[FeraligatrMaster]

Turtonator is not a pseudo-legendary.

Crobat, Ninjask, and Gliscor

 

[metroid711]

Ninjask is the only one that doesn't look like a bat.

Gyarados, Pelipper, and Swanna

 

[KNS27]

Gyarados only Pokemon on that list who get Mega Evo.

Charmander, Litten, Cyndaquil

 

[metroid711]

Cyndaquil, the only one to evolve into a mono fire type

Mismagius, Gengar, and Chandelure

 

[PageEmp]

Gengar doesnt require a stone to evolve.

Greninja, Blastoise, Clauncher

 

[faf]

Clauncher isn't part of a three stage family.
Pikachu, Jigglypuff, Mewtwo