I just noticed this, but the 3rd gen has the most "pairs" of Pokemon...

[stigy]

There's Plusle and Minun, Volbeat and Illimise, Lunatone and Solrock, Zangoose and Seviper, and Latias and Latios. I guess it's another way of indorsing those double battle thingys.

 

[jellsprout]

Latios and Latias.
Groudon and Kyogre.
Seedot and Lotad.
Beatifly and Dustox.
There probably are many more, but this is a good amount.
Yeah, seeing as how most of them learn double battle moves, as Helping Hand, I think that this is because of double battles.

 

[Silver Raven]

There's Plusle and Minun, Volbeat and Illimise, Lunatone and Solrock, and Zangoose and Seviper. I guess it's another way of indorsing those double battle thingys.

um.....there arch-enemies not friends.

lets see:

KANTO:

Hitmonlee/Hitmonchan/Hitmontop

JOTHO:

Umbreon/Espeon
Hitmonlee/Hitmonchan/Hitmontop

HOENN:

Plusle/Minun
Volbeat/Illimise
Lunatone/Solrock

yeah your right. i think its because of the double battles.

 

[AJ Flibble]

um.....there arch-enemies not friends.

lets see:

KANTO:

Hitmonlee/Hitmonchan/Hitmontop

JOTHO:

Umbreon/Espeon
Hitmonlee/Hitmonchan/Hitmontop

HOENN:

Plusle/Minun
Volbeat/Illimise
Lunatone/Solrock

yeah your right. i think its because of the double battles.

What abou Bellsprout/Oddish, Magmar/Electabuzz, all the version exclusives?

 

[Magma Grunt Alex]

dont forget Butterfree/Beedrill

 

[Espeon POWER!]

Yeah it's becuase it's a good way to get the double battles goin'

 

[killerjaw01]

Do Shuppet and Duskull count as a pair (or rival)? Anyway I think this makes it more interesting.

 

[Skittygirl]

What about
Plusle/Minun
Volbeat/Illumise
Beutifly/Dustox
Vileplume/Bellosom
Igglybuff/Cleefa

 

[Eerie Fuwante]

I like calling them opposites,instead of pairs.

Suppet/Duskull
Zangoose/Seviper(there opposites,not pairs)
Growlithe/Ponyta
Psyduck/Slowbro
Sneasel/Delibird
Lotad/Seedot
Plusle/Minun
Oddisg/Bellsprout
Anorith/Lileep
Omanyte/Kabuto
Mewtwo/Mew
Groudon/Kyogre
Lugia/Ho-Oh
Electabuzz/Magmar
Beutifly/Dustox
Sableye/Mawile
Hitmonlee/Hitmonchan
Espeon/Umbreon
Lunatone/Solrock

Those are some I can think of.

 

[Aikou Bupiggu]

I like calling them opposites,instead of pairs.

Suppet/Duskull
Zangoose/Seviper(there opposites,not pairs)
Growlithe/Ponyta
Psyduck/Slowbro
Sneasel/Delibird
Lotad/Seedot
Plusle/Minun
Oddisg/Bellsprout
Anorith/Lileep
Omanyte/Kabuto
Mewtwo/Mew
Groudon/Kyogre
Lugia/Ho-Oh
Electabuzz/Magmar
Beutifly/Dustox
Sableye/Mawile
Hitmonlee/Hitmonchan
Espeon/Umbreon
Lunatone/Solrock

Those are some I can think of.

Growlithe was actually opposed with Vulpix =P

If this was groupings, each evolution of Evee.

 

[Gravy]

Would Groudon and Kyogre really count? I know each respective pokémon is thought of as the 'feature' monster of each game, but personally I've always considered them to be part of a trio, due to Rayquazza's involvement.

 

[Evanji Axu]

You forgot Huntail/Gorebyss.

 

[[Kingdra]]

Scyther/Pinsir is another one

 

[~Shadow Espeon~]

You Guys Forgot About Groudan/kyogre!

 

[DJ Panic!]

i don't really point these things out but....

cool?

 

[Eerie Fuwante]

I like calling them opposites,instead of pairs.

Suppet/Duskull
Zangoose/Seviper(there opposites,not pairs)
Growlithe/Ponyta
Psyduck/Slowbro
Sneasel/Delibird
Lotad/Seedot
Plusle/Minun
Oddisg/Bellsprout
Anorith/Lileep
Omanyte/Kabuto
Mewtwo/Mew
Groudon/Kyogre
Lugia/Ho-Oh
Electabuzz/Magmar
Beutifly/Dustox
Sableye/Mawile
Hitmonlee/Hitmonchan
Espeon/Umbreon
Lunatone/Solrock

Those are some I can think of.

You didn't see it?

Suppet/Duskull
Zangoose/Seviper(there opposites,not pairs)
Growlithe/Ponyta
Psyduck/Slowbro
Sneasel/Delibird
Lotad/Seedot
Plusle/Minun
Oddisg/Bellsprout
Anorith/Lileep
Omanyte/Kabuto
Mewtwo/Mew
Groudon/Kyogre
Lugia/Ho-Oh
Electabuzz/Magmar
Beutifly/Dustox
Sableye/Mawile
Hitmonlee/Hitmonchan
Espeon/Umbreon
Lunatone/Solrock
Pinsir/Heracross
Pinsir/Scyther
Huntail/Gorebyss
Sandshrew/Ekans
Spearow/Pidgey
Sentret/Zigzagoon
Butterfly/Beedrill
Magnemite/Voltorb

Added some more.

 

[Selestius]

Ledyba/Spinarak
Ledian/Ariados
Caterpie/Weedle
Metapod/Kakuna
Nidoran M/Nidoran F
Nidorino/Nidorina
Nidoking/Nidoqueen
Grimer/Koffing [RS, anyone?]
Phanpy/Teddiursa
Donphan/Ursaring
Gligar/Skarmory

That's all I can think of.

 

[PDL]

pairs? lesse...

RBY/FRLG

Caterpie/Weedle (add wurmple)
Pidgey/Spearow (add Hoothoot and Wingull/Taillow)
Ekans/Sandshrew
NidoranF/NidoranM
Clefairy/Jigglypuff (add Skitty or even Snubull)
Vulpix/Growlithe (add Houndour)
Oddish/Bellsprout
Meowth/Mankey
Paras/Venonat
Abra/Machop/Geodude/Gastly (more of a foursome then a twosome)
Psyduck/Slowpoke (FR/LG)
Magnamite/Voltorb
Grimer/Koffing
Shellder/Staryu (FR/LG)
Hitmonlee/Hitmonchan (add Hitmontop and/or Tyrouge)
Mr.Mime/Jynx (add with Electabuzz/Magmar)
Scyther/Pinsir
Electabuzz/Magmar (add with Jynx, maybe Mr.Mime)
Omanyte/Kabuto (add with Lileep/Anorith)
Mewtwo/Mew (?)

GSC

Rattata/Sentret
Ledyba/Spinarak
Vileplume/Bellossom
Poliwarth/Poiltoed
Marill/Snubbull (GSC)
Marill/Wooper (FR/LG)
Yanma/Dunsparce (GSC... sorta)
Espeon/Umbreon
Murkrow/Misdreavus (FR/LG)
Snorlax/Sudowoodo
Slowbro/Slowking
Misdreavus/Sneasel (maybe)
Gligar/Delibird (GSC)
Qwilfish/Remoraid
Scizor/Heracross (or Scizor/Scyther or Heracross/Pinsir)
Snubbull/Clefairy (or Snubull/(Skitty/Jigglypuff)
Teddiusra/Phanpy
Delibird/Sneasel (FR/LG)
Mantine/Skarmory
Smeargle/Mr.Mime (maybe)
Miltank/Blissey
Dratini/Larvitar (add Bagon/Beldum)
Lugia/Ho-oh

RSE

Poochyena/Zigzagoon
Beautifly/Dustox
Lotad/Seedot
Taillow/Wingull
Sirskit/Nincada
Makuhita/Meditite
Skitty/Clefairy (or Skitty/Jigglypuff)
Sableye/Mawile
Plusle/Minun
Volbeat/Illumise
Carvanha/Wailmer (?)
Wailmer/Numel
Zangoose/Seviper
Lunatone/Solrock
Lileep/Anorith (add the Kanto fossils if you want)
Shuppet/Duskull
Huntail/Gorebyss
Bagon/Beldum (add with Dratini/Larvitar)
Latias/Latios
Groudon/Kyogre (add Rayquaza if you want)

yeah, there are quite a few pairs or threesomes, even whole groups if you look hard enough. It's not just types and evolutions that connect pokemon.

 

[~Shadow Espeon~]

Ledian and Ariados
Dustox and Beautifly
Houndoom and Mightyena
Pidgeot and Fearow???
Nidoking and Nidoqueen(duh)
Jynx and Mr. Mime
Furret and Linoone

Heres some but I think people have already submitted all these pairs.

 

[AJ Flibble]

Ratatta/Pidgey
Sentret/Hoothoot
Ekans/Sandshrew
Dewgong/Cloyster/Lapras (I think because they're all Water/Ice Kanto, maybe the Spheal line could come in here for Water/Ice overall)
Steelix/Scizor (Metal Coat link)
Politoed/Slowking (both Kings Rock, both govern their species)
Articuno/Zapdos/Moltres (and Lugia and Ho-oh too)
Kingdra/Dragonite (only Dragons in kanto and Johto respectively)
Miltank/Tauros
Tauros/Chansey (Safari zone, hard to find and catch)
Magikarp/Feebas
All Hitmons and Tyrogue
Heracross/Scizor (powerful Johto bugs)
Shuckle/Aipom (both found in unusual ways, both unusual Pokemon)
Gengar/Misdreavus (see Dragonite/kingdra, but for Ghost)
Bellossom/Sunflora (Sun Stone)
Eeveelutions and Eevee
Umbreon/Absol/Mightyena (pure Darks, all mammals)