Odd One Out

[Sonata]

Cyndaquil because it's the only one that isn't a gen 3 starter.

Swampert, Feraligatr, Blastoise

 

[Juno]

Blastoise is the only one with a Mega form.

Carbink, Klefki, Dedenne?

 

[classiccartoonsftw]

Dedenne is weak to Poison.

Forretress, Ferrothorn, and Aggron

 

[Nah]

Forretress, since its name doesn't have anything to do with iron (Aggron comes straight from iron, while the ferro in Ferrothorn is the Latin word for iron, iirc. Forretress comes more from fortress).

Tyranitar, Garchomp, Metagross

 

[orangeralts21]

Metagross is the only one without a 4x weakness(Tyrannitar is 4x weak to fighting and Garchomp is 4x weak to ice)

Cacturne, Flygon and Sableye

 

[Sonata]

Sableye is the only one that has no prior evolutions.

Raticate, Linoone, Furret.

 

[isthatyoudan]

Raticate because he is short and fat while the other two are long and skinny.

Pansear, Vanillite, Luxray.

I think this is a toughie.

 

[Khoshi]

Pansear doesn't get any Ice-type moves. Luxray has Ice Fang, and Vanillite is self-explanatory. :p

Greninja, Kecleon, Porygon2.

 

[Juno]

Kecleon is not part of any evolutionary line.

Cherubi, Cacnea, Shroomish.

 

[Khoshi]

Out of the 3, I believe Cherubi's evolution doesn't become dual typed.

Feebas, Tympole, Unown.

 

[Nah]

Unown is not an aquatic creature. It's also the only one that has multiple forms and learns Hidden Power naturally.

Klink, Rotom-W, Muk

 

[waveguidebuizel]

Muk, because it's not really solid.

Pancham, Sandslash, Dewott.

 

[Voliol]

Dewott, it doesn't have multiple A:s in its name.

Oddish, Gloom and Vileploom!

 

[Khoshi]

Oddish isn't a flower thing, instead it's a weed.

Deoxys, Vivillon, Forretress.

 

[Fernbutter]

Deoxys is a space alien thing. The other two are actually based off of real things.

What about Charmeleon, Quilava, and Kombusken?

 

[Juno]

Combusken is the only dual-type Pokemon of the three.

Escavalier, Scizor, Forretress?

 

[Candy]

Forretress is the only one that doesn't evolve through trade.

Igglybuff, Cleffa, Azurill

 

[Khoshi]

Azurill doesn't have Cute Charm.

Geodude, Pineco, Skarmory.

 

[Nah]

Skarmory is the only one that can't use Self-Destruct/Explosion.

Gardevoir, Sylveon, Granbull

 

[CliCliW]

Sylveon is the only one that didn't get re-typed to Fairy in 6th Gen (cause it was always Fairy)

Electabuzz, Scyther, Jynx