Odd One Out

[Nah]

Wingull's not based off of a duck.

Rattatta, Patrat, Zigzagoon.

 

[Hiatus]

Zigzagoon is the only one that doesn't have "rat" in its name.

Pidgeot, Staraptor, Pelipper.

 

[destinedjagold]

Pidgeot only has two syllables (at least, the way I say its name), ends with a 't', and has three different vowels in its name. :P

Spearow, Hoothoot, and Taillow

 

[Imperator161]

Spearow doesn't have any double letters.

Weezing, Tangela, and Mr. Mime.

 

[Apollo]

Weezing never received an evolution or a pre-evo in Gen. 4.

Dedenne, Lapras, Roselia

 

[RuFF]

Dedenne. It doesnt have a letter A in its name.

YANMEGA, GLISCOR, SWELLOW

 

[Lenny ( ͡° ͜ʖ ͡°)]

Yanmega, it doesn't have a Y in it's name.

BIDOOF, BIBAREL, BALTOY

 

[Renegade]

Baltoy isn't an evolution of the Bidoof family.
Baltoy is specific to desert areas while the Bidoof family isn't.

Unown, Sceptile, & Zebstrika~

 

[Khoshi]

Unown isn't based on an animal. Obvious, I know.

Sliggoo, Slugma, Muk.

 

[Nah]

Muk isn't based off of a slug.

Heatran, Arcanine, Typholsion

 

[Juno]

Heatran is not a part of any evolutionary line.

Beldum, Aron and Honedge?

 

[Hiatus]

Honedge's evolution has two forms, while other two doesn't.

Squirtle, Wingull, and Tentacool?

 

[Nah]

Squirtle, since it doesn't constantly pop up on Hoenn's water routes.

Zubat, Roggenrola, Drilbur

 

[Khoshi]

Roggenrola isn't organic - it's made up of inanimate objects.

Skitty, Pyroar, Liepard.

 

[Ghost Lunis]

Skitty isn't an evolved form

Plusle, minun, emolga

 

[Renegade]

Emolga is duel-typed, the other two are not.

Torterra, Ghastly, & Magmar~

 

[Juno]

Both Magmar and Ghastly are part of the original 150/151 Pokemon, while Torterra appeared Gen. IV onwards.

Sandshrew, Diglett, Cubone?

 

[Khoshi]

Cubone can't learn Slash, while Sandshrew can and Diglett's sort of famous for it. Yay Gen 1 mechanics.

Hariyama, Snorlax, Mienshao.

 

[Shadow Man]

Besides the obvious "Snorlax isn't fighting", Mienshao and Hariyama are Stage 1s who don't have a baby form.

Gliscor, Vileplume, Politoed.

 

[Khoshi]

Politoed's ability doesn't activate in weather - it is the weather. :P (to clarify: Politoed's ability doesn't need weather to activate, while the other 2 do need weather).

Ninetales, Lapras, Pikachu.