For simplicity's sake, a list of the Pokémon NOT appearing in X and Y would be useful.
We still have 29 holes to plug in the Regional Dex (3 of which may include Torchic, Combusken and Blaziken... but probably not).
Here's
what isn't available in Pokemon X & Y as of right now...
KANTO - GEN 1
Rattata / Raticate
Spearow / Fearow
Clefairy / Clefable (you'd think otherwise given this is the Fairy gen - but it's not on any Regional Dex yet)
Vulpix / Ninetales
Paras / Parasect
Venonat / Venomoth
Meowth / Persian (seriously, milking the blatant nostalgia game and no Meowth?)
Mankey / Primeape
Growlithe / Arcanine
Ponyta / Rapidash
Seel / Dewgong
Grimer / Muk
Drowzee / Hypno
Krabby / Kingler
Voltorb / Electrode
Hitmonlee / Hitmonchan
Lickitung
Koffing / Weezing
Chansey
Tangela (this guy's always getting passed over)
Electabuzz
Magmar
Porygon
Omanyte / Omastar
Kabuto / Kabutops
Articuno (already confirmed to be roaming around post-game; safe to assume all three of these birds can be acquired)
Zapdos (see above)
Moltres (see above)
Dratini / Dragonair / Dragonite
Mew
JOHTO - GEN 2
Chikorita / Bayleef / Meganium
Cyndaquil / Quilava / Typhlosion
Totodile / Croconaw / Feraligatr
Cleffa
Togepi / Togetic (supposedly already found and confirmed as a Fairy-type; still awaiting a Regional Dex number)
Natu / Xatu
Sudowoodo
Aipom
Sunkern / Sunflora
Misdreavus
Unown
Girafarig
Pineco / Forretress
Shuckle
Corsola
Phanpy / Donphan
Porygon 2
Stantler
Tyrogue / Hitmontop
Elekid
Magby
Blissey
Raikou
Entei
Suicune
Lugia
Ho-oh
Celebi
HOENN - GEN 3
Treecko / Grovyle / Sceptile
Mudkip / Marshtomp / Swampert
Wurmple / Silcoon / Beautifly / Cascoon / Dustox
Seedot / Nuzleaf / Shiftry
Taillow / Swellow
Shroomish / Breloom
Slakoth / Vigoroth / Slaking
Aron / Lairon / Aggron
Numel / Camerupt
Spinda
Cacnea / Cacturne
Baltoy / Claydol
Lileep / Cradily
Anorith / Armaldo
Feebas / Milotic
Castform
Duskull / Dusclops
Tropius
Snorunt / Glalie
Spheal / Sealeo / Walrein
Clamperl / Huntail / Gorebyss
Beldum / Metang / Metagross
Regirock
Regice
Registeel
Latias
Latios
Kyogre
Groudon
Rayquaza
Jirachi
Deoxys
SINNOH - GEN 4
Turtwig / Grotle / Torterra
Chimchar / Monferno / Infernape
Piplup / Prinplup / Empoleon
Starly / Staravia / Staraptor
Kricketot / Kricketune
Shinx / Luxio / Luxray
Cranidos / Rampardos
Shieldon / Bastiodon
Cherubi / Cherrim
Shellos / Gastrodon
Ambipom
Buneary / Lopunny
Mismagius
Glameow / Purugly
Stunky / Skuntank
Bronzor / Bronzong
Bonsly
Happiny
Spiritomb
Finneon / Lumineon
Lickilicky
Tangrowth (once again, this guy's always getting passed over)
Electivire
Magmortar
Togekiss (once again, supposedly already found and confirmed as a Fairy-type; still awaiting a Regional Dex number)
Porygon-Z
Dusknoir
Froslass
Rotom
Uxie
Mesprit
Azelf
Dialga
Palkia
Heatran
Regigigas
Giratina
Cresselia
Phione / Manaphy
Darkrai
Shaymin
Arceus
UNOVA - GEN 5
Victini
Snivy / Servine / Serperior
Tepig / Pignite / Emboar
Oshawott / Dewott / Samurott
Lillipup / Herdier / Stoutland
Munna / Musharna
Pidove / Tranquill / Unfezant
Blitzle / Zebstrika
Drilbur / Excadrill
Tympole / Palpitoad / Seismitoad
Sewaddle / Swadloon / Leavanny
Cottonee / Whimsicott
Petilil / Lilligant
Darumaka / Darmanitan
Maractus
Yamask / Cofagrigus
Tirtouga / Carracosta
Archen / Archeops
Minccino / Cinccino
Deerling / Sawsbuck
Frillish / Jellicent
Alomomola
Joltik / Galvantula (a long-rumored Mega that hasn't even shown up in Dex form)
Klink / Klang / Klinklang
Tynamo / Eelektrik / Eelektross
Elgyem / Beheeyem
Bouffalant
Rufflet / Braviary
Vullaby / Mandibuzz
Larvesta / Volcarona
Cobalion
Terrakion
Virizion
Tornadus
Thundurus
Reshiram
Zekrom
Landorus
Kyurem
Keldeo
Meloetta
Genosect