Odd One Out

[Juno]

I'm not too caught up on the anime as of late, but I'm pretty sure Ash never had a Ninetales.
If that is a lie, my backup answer is Ninetales has a gender ratio of 1 male:3 females.

Caterpie, Wurmple and Weedle?

 

[Apollo]

Caterpie doesn't have a poison-type evolution.

Xatu, Honedge, Igglybuff

 

[Hiatus]

Honedge, as it's the only one that has metallic body structure.

Tailow, Pidgey, and Wingull.

 

[Juno]

Pidgey is part of a three-stage evolutionary line, while Taillow and Wingull both only evolve once.

Glaceon, Espeon and Jolteon?

 

[eeveeandmew]

Jolteon, as it is not from a non kanto region.

Eevee, mew, sandile

 

[Juno]

Mew is the only one who does not have any type immunities - Eevee is immune to Ghost while Sandile is immune to Psychic and Electric.

Ninetales, Arcanine, and Flareon.

 

[Khoshi]

Ninetales doesn't have any fur around its head, unlike the others.

Ho-oh, Zekrom, Caterpie.

 

[Nah]

Zekrom, because it wrecks the other two.

Banette, Sableye, Dusclops

 

[Brolijah]

Dusclops because it doesn't ever receive Prankster.

Fennekin, Zorua, Riolu.

 

[Aisu]

Unlike Fennekin and Zorua, Riolu has no hair to spare! Now, Heliolisk, Diggersby, and Pikachu.

 

[Sapph the Knight]

Pikachu, it's not part normal-type.

Charizard, Typhlosion, Emboar

 

[Juno]

Charizard is the only one who has been a version mascot.

Floette, Togetic and Roselia?

 

[brianwee2012]

Floette, Togetic and Roselia
Roselia is the only non fairy type pokemon.

Charizard, Aggron, Garchomp

 

[RuFF]

Charizard - He is the only one with two megaevolutions

Politoed = Slowking = Kingdra (Not in the name)

 

[Nah]

Slowking doesn't have a Rain related ability.

Ho-oh, Slowbro, Dedenne

 

[LeGreyfish]

Dedenne, because it cannot be obtained in GSC or HGSS.

Venusaur, Meganium, Sceptile. (No it's not typing.)

 

[classiccartoonsftw]

Sceptile stands on two legs instead of four.

Blastoise, Feraligatr, and Swampert (Anything except typing)

 

[Apollo]

The rival never uses a Swampert.

Zigzagoon, Cascoon, Absol

 

[Juno]

Absol has a Mega form.

Sableye, Pawniard, Sneasel.

 

[brianwee2012]

Sableye, Pawniard, Sneasel.

Sableye is the only one with a mega form. (FYI was released as a mega pokemon a few weeks ago)
--OR--
Sableye doesnt have an evolution.