I work for Game Freak as a developer, and I got to test the demo for Sun and Moon. The game's finished, but there might be some slight changes in the future.

Alolan forms are only for first generation Pokèmon, but not every Pokèmon has one. Arbok, Golem, Machamp, Starmie, Dragonite, Marowak, Raichu, Raticate, Farfech'd, Beedrill, and Butterfree have Alola forms, along with some more. Kingdra, Toxicroak, Tropius, Milotic, Bisharp, Druddigon and Froslass also have Mega Evolutions now as far as I've seen.

The Pokèdex will be split into 5 different parts (one for each island, and one for all of Alola). Each island has 20-30 native Pokèmon exclusive to itself (Not including Alola forms).

Game Freak wants Sun and Moon to be appealing to casual and hardcore gamers, so there aren't as many new Pokèmon in this generation compared to previous games because almost all Pokèmon from Generation 1 are in Alola as well.

Velopter, Velourus, and Velocerex are dragon/fairy types exclusive to Pokèmon Sun. Velopter is based off a Veloceraptor.

Flarseed will evolve into either a Tulip, Sunflower, or Daisy depending if it is dawn, mid-day, or dusk. It is based off of a flaming seed.

Mowseal is a water type, and Monoseal and Sealord are water/poison types. Exclusive in Pokèmon Moon. Sealord is based off of the Northern elephant seal.

Rodenlant, Ratintant
Rodenlant is a ground type, while his evolution adds on a Dark Typing. Remodenlant and Ratintant iare based off of rats.

The playable demo is planned to be in the 3DS eShop mid-October.