So, what's in the Alola Dex? (As of 10/5/2016)

Update 10/6/2016: Missed Gothitelle line on clothing.
Update 10/14/2016: Alolan Grimer/Muk

Before XY was released, TPC/TPCi ended up showing a bunch of non-Kalos Pokémon in their trailers and other pre-release footage. With the exception of Blaziken, every Pokémon shown ended up being in the Kalos dex. The leadup to Sun and Moon has been very similar, and so I believe we can reasonably expect that most Pokémon that have been seen in trailers and/or official art will be part of the Alola Dex.

==Guaranteed or extremely likely==

This group includes most Pokémon that have been physically seen in trailers and E3 for the game. Most of these evolutionary families have had members appear in multiple trailers.

Caterpie/Metapod/Butterfree
Rattata/Raticate (Alola Form)
Spearow/Fearow
Pichu/Pikachu/Raichu (Alola Form (Raichu))
Sandshrew/Sandslash (Alola Form)
Cleffa/Clefairy/Clefable
Vulpix/Ninetales (Alola Form)
Igglybuff/Jigglypuff/Wigglytuff
Zubat/Golbat/Crobat
Paras/Parasect
Meowth/Persian (Alola Form)
Psyduck/Golduck
Mankey/Primeape
Growlithe/Arcanine
Poliwag/Poliwhirl/Poliwrath/Politoed
Abra/Kadabra/Alakazam
Machop/Machoke/Machamp
Slowpoke/Slowbro/Sloking
Magnemite/Magneton/Magnezone
Grimer/Muk (Alola Form)
Shellder/Cloyster
Gastly/Haunter/Gengar
Drowzee/Hypno
Exeggcute/Exeggutor (Alola Form (Exeggutor))
Cubone/Marowak (Alola Form (Marowak))
Happiny/Chansey/Blissey
Kangaskhan
Goldeen/Seaking
Staryu/Starmie
Scyther/Scizor
Elekid/Electabuzz/Electivire
Magby/Magmar/Magmortar
Pinsir
Tauros
Magikarp/Gyarados
Lapras
Ditto
Eevee/Vaporeon/Jolteon/Flareon/Espeon/Umbreon/Leafeon/Glaceon/Sylveon
Aerodactyl
Munchlax/Snorlax
Dratini/Dragonair/Dragonite

Ledyba/Ledian
Spinarak/Ariados
Bonsly/Sudowoodo
Murkrow/Honchkrow
Misdreavus/Mismagius
Snubbull/Granbull
Sneasel/Weavile
Corsola
Delibird
Skarmory
Miltank

Wingull/Pelipper
Surskit/Masquerain
Shroomish/Breloom
Makuhita/Hariyama
Nosepass/Probopass
Torkoal
Trapinch/Vibrava/Flygon
Feebas/Milotic
Absol
Luvdisc
Bagon/Shelgon/Salamence
Beldum/Metang/Metagross

Shieldon/Bastiodon
Drifloon/Drifblim
Gible/Gabite/Garchomp
Riolu/Lucario
Finneon/Lumineon
Rotom

Lillipup/Herdier/Stoutland
Roggenrola/Boldore/Gigalith
Petilil/Lilligant
Sandile/Krokorok/Krookodile
Tirtouga/Carracosta
Archen/Archeops
Trubbish/Garbodor
Emolga
Rufflet/Braviary

Fletchling/Fletchinder/Talonflame
Pancham/Pangoro
Carbink
Goomy/Sliggoo/Goodra
Phantump/Trevenant
Zygarde

And of course, all of the new Gen 7 Pokémon.

So assuming these are all correct, we know over 200 of the Pokémon in the Alola Dex.

Now for Pokémon that have a reasonable shot of being in the Dex or were shown in trailers, but aren't as likely as these:

==Poké Rides==
Charmander/Charmeleon/Charizard
Carvanha/Sharpedo

Sharpedo is probably in the Dex, but Charizard would be surprising without Venusaur and Blastoise.

==Crabrawler Reveal Trailer (Worlds)==
Smeargle
Cresselia
Ferroseed/Ferrothorn
Landorus

The Pokémon shown in this trailer were specifically chosen due to their VGC presence, so I think they should probably be taken with a grain of salt, particularly Cresselia/Landorus.

==Buildings/Clothing/Signage/Other Structures==
Krabby/Kingler
Horsea/Seadra/Kingdra
Chinchou/Lanturn
Wailmer/Wailord
Swablu/Altaria
Barboach/Whiscash
Clamperl/Huntail/Gorebyss
Gothita/Gothorita/Gothitelle

Likenesses of these Pokémon have been incorporated into signs, fountains, boats, etc. Past experience suggests that they're probably in the Dex, but it's not guaranteed until they're seen.

==Pokémon Refresh==
Porygon/Porygon2/Porygon-Z
Larvitar/Pupitar/Tyranitar

Likenesses of these Pokémon are incorporated into some of the grooming items in Refresh. Possible, but not enough information right now.

==Ash-Greninja==
Froakie/Frogadier/Greninja

I don't think Greninja will be in the Alola Dex. This feels similar to the Mega Blaziken promotion for XY.

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I think that's it! Let me know @violetpumpkin_ if I'm missing anything or if you want to know where something was shown!