Serebii has a list of the specific Pokemon you cannot capture in Colosseum, these are typically tutorial battles/gym battles/challenge battles/boss battles or any trainer battle without a shadow Pokemon. where you cant use a Snag Ball.
Bulbapedia has a list of all Pokemon obtainable in the game.
Now all you would have to do is go through eight pages on Serebii and cross check between Bulba to get your answer <:D but that would take too long and because I'm a real helpful guy I'll do it for everyone <:D
Pokemon species not obtainble in Colosseum that can still be normally found through regular gameplay:
- Zigzagoon & Linoone
- Whismur, Loudred & Exploud
- Sentret
- Noctowl
- Wurmple, Silcoon, Cascoon, Beautifly & Dustox
- Corphish & Crawdaunt
- Koffing & Weezing
- Spinarak & Ariados
- Duskull & Dusclops
- Spoink & Grumpig
- Hoppip
- Sunkern
- Azurill, Marril & Azumarill
- Surskit & Masquerain
- Trapinch
- Swinub
- Rhyhorn & Rhydon
- Geodude, Graveler & Golem
- Tailow & Swellow
- Slakoth, Vigoroth, Slaking
- Zubat, Golbat & Crobat
- Gulpin & Swalot
- Skitty & Delcatty
- Shuppet & Banette
- Igglybuff, Jigglypuff & Wigglytuff
- Numel & Camerupt
- Ledyba
- Wingull & Pelliper
- Doduo & Dodrio
- Shroomish & Breloom
- Pichu, Pikachu (although available in Japanese bonus disc, you can only send it to Ruby or Sapphire) & Raichu
- Oddish, Gloom, Vileplume & Bellossom
- Psyduck & Golduck
- Horsea, Seadra & Kingdra
- Ralts, Kirlia, & Gardevoir
- Machop, Machoke & Machamp
- Seedot, Nuzleaf & Shiftry
- Snubbull
- Sandshrew & Sandslash
- Barboach & Whiscash
- Natu & Xatu
- Elektrike & Manectric
- Cacnea & Cacturne
- Vulpix & Ninetales
- Goldeen & Seaking
- Bagon, Shelgon & Salamence
- Magnemite & Magneton
- Pineco
- Nincada, Ninjask & Shedninja
- Phanpy & Donphan
- Larvitar & Pupitar
- Carvanha & Sharpedo
- Wooper
- Voltorb & Electrode
- Spheal, Sealeo & Walrein
- Snorunt & Glalie
- Tentacool & Tentacruel
- Luvdisc
- Anorith & Armaldo
- Lotad, Lombre & Ludicolo
- Whismur, Loudred & Exploud
- Mareep (available only through Japanese e-reader)
- Beldum & Metang
- Teddiursa
- Lileep & Cradily
- Baltoy & Claydol
- Wynaut & Wobbuffet
- Clamperl & Huntail (Gorebyss not in the game)
- Houndour
- Spinda
- Kadabra & Alakazam
- Marshtomp & Swampert
- Milotic
- Magikarp & Gyarados
- Wailmer & Wailord
- Illumise
- Steelix
- Roselia
- Seviper
- Corsola
- Castform
- Chinchou & Lanturn
- Grovyle & Sceptile
- Kecleon
- Muk
- Starmie
- Aron, Lairon & Aggron
- Mawile
- Chimeco
- Grumpig
- Porygon2
- Zangoose
- Mightyena
- Torkoal
- Combusken & Blaziken
- Scizor (available only through Japanese e-reader)
- Slowking
- Volbeat
- Girafarig
- Solrock
- Blissey
- Relicanth
- Sableye
Pokemon such as Porygon who are not available in the game, but who's evolution are I have left out btw.
Now for Pokemon XD, I used this Serebii list to figure out who is obtainable, but there is not a list like the list above that shows every Pokemon who is unobtainable, so I had to look at all battles and figure out who is not catchable. I did that through here.
- Sentret & Furret
- Sunkern & Sunflora
- Magikarp & Gyarados
- Abra, Kadabra & Alakazam
- Whismur, Loudred & Exploud,
- Azurill, Marill & Azumarill
- Taillow
- Zubat, Golbat & Crobat
- Wingull & Pelliper
- Lotad, Lombre & Ludicolo
- Swablu
- Feebas & Milotic
- Oddish, Gloom, Vileplume & Bellossom
- Machop, Machoke & Machamp
- Wurmple, Silcoon, Cascoon Beautifly & Dustox
- Igglybuff, Jigglypuff & Wigglytuff
- Nincada, Ninjask & Shedninja
- Tyrogue & Hitmontop (Hitmonchan and Hitmonlee are available)
- Zigzagoon & Linoone
- Snubbull & Granbull
- Cacnea & Cacturne
- Corphish & Crawdaunt
- Slugma
- Horsea, Seadra & Kingdra
- Pichu, Pikachu & Raichu
- Doduo
- Spoink & Grumpig
- Hoothoot & Noctowl
- Elektrike
- Chinchou & Lanturn
- Goldeen & Seaking
- Koffing & Weezing
- Tentacool & Tentacruel
- Skitty
- Anorith & Armaldo
- Lileep & Cradily
- Shuppet
- Bagon & Shelgon
- Barboach & Whiscash
- Staryu
- Wailmer & Wailord
- Wynaut & Wobbuffet
- Corsola
- Qwilfish
- Magnemite
- Psyduck
- Remoraid & Octillery
- Chimeco
- Dunsparce
- Kecleon
- Clamperl, Gorebyss & Huntail
- Murkrow
- Rhyhorn
- Slakoth, Vigoroth & Slaking
- Beldum, Metang & Metagross
- Luvdisc
- Aipom
- Surskit & Masquerain
- Delibird
- Sneasal
- Yanma
- Misdreavous
- Seviper
- Absol
- Sudowoodo
- Volbeat
- Illumise
- Torkoal
- Stantler
- Geodude, Graveler & Golem
- Wooper & Quagsire
- Castform
- Spinda
- Hoppip, Skiploom & Jumpluff
- Tropius
- Plusle
- Minun
- Sandshrew & Sandslash
- Gligar
- Relicanth
- Smeargle
- Mantine
- Skarmory
- Trapinch, Vibrava & Flygon
- Slowpoke, Slowbro & Slowking
- Aron, Lairon & Aggron
- Meditite & Medicham
- Shuckle
- Heracross
- Phanpy & Donphan
- Miltank
- Aerodactyl
- Smoochum & Jynx
- Cleffa, Clefairy & Clefable
- Torchic, Combusken & Blaziken
- Larvitar, Pupitar & Tyranitar
- Marshtomp & Swampert
- Nidoking
- Kingler
- Ponyta
- Kabutops
- Ditto
- Gengar
- Regice
- Regirock
- Nidoqueen
- Politoed
- Steelix
- Raikou
- Sceptile
- Charizard
- Venusaur
- Blastoise
- Entei
- Omastar
- Suicune
- Latias
- Latios
