I wouldn't necessarily say Genesect is going to be catchable in game.
But here's the list:
Pikachu
Magikarp
Golurk
Munchlax
Kirlia
Pansear
Patrat
Litwick
Psyduck
Audino
Weavile
Hariyama
Gligar
Krokorok
Genesect
Rampardos
Cloyster
Gyarados
Machamp
Zangoose
Presumably their evos will be in there too, by intuition.
Thanks for the list. Well yes, barring Genesect since it was alluding to the movie. So these would presumably obtainable in-game in one way or another:
Pikachu (Pichu & Raichu)
Magikarp & Gyarados
Golurk (Golett)
Munchlax (Snorlax)
Kirlia (Ralts, Gardevoir, & Gallade)
Pansear (Simisear) [if they have this guy, then the other "Pans-" does seem likely to appear in the game]
Patrat (Watchog)
Litwick (Lampent & Chandelure)
Psyduck (Golduck)
Audino
Weavile (Sneasel)
Hariyama (Makuhita)
Gligar (Gliscor)
Krokorok (Sandile & Krookodile)
Rampardos (Cranidos)
Cloyster (Shellser)
Machamp (Machop & Machoke)
Zangoose [Seviper is like the opposite of Zangoose so this wouldn't be unexpected]
So I'm counting 45 Pokemon that are from previous gens can possibly be possibly be obtainable in the game from what has been revealed so far.