Odd One Out

[Tsutarja]

Onix has no legs :P

Steelix, Scizor, Flaafy

 

[Emboar]

Scizor isn't weak to Ground.

Heracross, Togekiss, Magmar.

 

[Sirfetch’d]

Heracross isn't an evolution!

Vivillon, Butterfree, Beautifly?

 

[Hikamaru]

Beautifly's the only one not to have Compound Eyes as an ability.

Drifloon, Venipede, Inkay

 

[Return]

Venipede is the only Pokemon there which has three evolution stage rather than two!

Alakazam,Emolga,Bayleef

 

[Hikamaru]

The other answer I would have accepted was Inkay was the only one to not have a Japanese name starting with F. Anyways, Emolga is not part of an evolution line.

Psyduck, Doduo, Vulpix

 

[Who's Kiyo?]

Vulpix's PokeDex number is odd, while the other two have even numbers that end with the number 4.

Roggenrola, Geodude, Onix​

 

[bradleyklaw]

Geodude is the only one out of the three with arms!

Rattata, Sentret, and Zigzagoon.

 

[Nidorina of Time Lord]

Rattata doesn't have any sort of stripes in its design.

Infernape, Emboar and Blaziken

 

[Abby]

Emboar is the only one to get a non hiuden power water move.

Shiftry, zygarde and chatot

 

[Who's Kiyo?]

Chatot is the only Pokemon who could be interacted with or affected by the built-in DS microphone in the Generation 4 games via its access to the move Chatter.

Ludicolo, Raichu, Crobat.​

 

[Sirfetch’d]

Crobat is the only one immune to Earthquake!

Stantler, Delibird, Jynx?

 

[Who's Kiyo?]

Delibird is the only Pokemon that has the same name in Japanese as it does in English.

Goldeen, Golbat, Golduck?​

 

[Return]

Golbat is the only poison type Pokemon there.

Jolteon,Pikachu,Joltik

 

[Starry Windy]

Joltik is the only one that origins from Unova.

Qwilfish, Remoraid, Basculin.

 

[Nidorina of Time Lord]

Basculin is not from Johto.

Pidgey, Pidove and Starly

 

[Return]

Pidgey is the only Pokemon that further evolution can mega evolve.

Chikorita,Oddish,Foongus

 

[Sirfetch’d]

Chikorita isn't part Poison!

Doublade, Metang, Fraxure?

 

[Who's Kiyo?]

Fraxure has two different unhidden abilities to vary from, while the others only have one.

Whimsicott, Tentacruel, Landorus?​

 

[Starry Windy]

Tentacruel is not originated from Unova. Or maybe Landorus, because it's the only Legendary Pokémon.

Cyndaquil, Torchic, Treecko.