Odd One Out

Sceptile is the only one not learning Synthesis through levelling.

Magcargo, Mamoswine, Magnezone

Mamoswine's name doesn't start with Mag.

Milotic, Mightyena, Misdreavus

Misdreavus is not in the Hoenn Pokedex.

Chinchou, Chingling, Chimchar.

Chimchar, its Japanese name doesn't have a long sound (ー).

Emboar, Giratina, Staravia

Dialga's the only one without a Fire-type weakness, as it's neutral.

Azurill, Marill, Azumarill

Kyurem is the only legendary

Eelektross, Raichu, Magnezone

Magnezone lacks feet of some sort.

Goldeen, Vulpix, Trubbish

Pangoro is the only one who's not from Hoenn.

Jynx, Glaceon, Jigglypuff

Eevee has a higher chance to be male than to be female.

Jolteon, Snivy, Relicanth

Relicanth is a dual type.

Druddigon, Haxorus, Dragonair

This is a hard one but still, I think Sceptile is odd one as it is the only one belonging to a starter family and is the only one whose mega evolution's type is different from original....

Find odd one: Spearow, Taillow, Hoothoot

Tailow, it's the only one to learn a fighting move (Quick Guard) naturally

Aipom, Mankey, Monferno

Starmie, it has no eyes.

Zubat, Bellossom, Luvdisc

Zubat, for the same reason given above i.e. no eyes while other two have eyes.....

Zubat, Golbat, Crobat

Bidoof, as it is the only one without a 4x weakness. :3

Honchcrow, Cinccino, Watchog

Honchkrow is the only one that isn't a normal type.

Weedle, Wurmple, Caterpie

Caterpie doesn't learn Poison Sting.

Braviary, Doublade, Xerneas.

Doublade sin't an animal based desing.

Poochyenna, weepinbell, mewtwo

Rhyperior don'T have a mega.

Flygon, Goodra, Charizard

Rhyperior is the only one to evolve through trading. Further more, it's the only one that looks like a turd (has 'corn' pieces on its 'armour'), doesn't have a Mega Evolution, and the only one that should never have ever seen the light of day. Too slow, dang. My assessment of Rhyperior remains unchanged, however.

-Silly Goodra, you can't fly!
*Goodra attempts to Fly*
-That wasn't flying! That was falling, with style!

There is a number of ways this list could have gone:
Goodra is the only single typed Pokemon on the list.
Charizard isn't a Dragon Type.
Charizard if the only Fire type
Charizard is the only Flying type
Goodra doesn't evolve through leveling up alone (must be rainy in the overworld)
Flygon is the only one immune to Electric type moves
Flygon cannot learn Outrage etc.

This is fun.

My list: Girafarig, Igglybuff, Avalugg