Alolan forms are just alternate versions of specific existing kanto mons that carry different movesets and typing. Some may also differ in terms of stats, but I don't really remember.
Most are "alolan" from the start (example: Alolan Muk evolves from Alolan Grimer, not normal Grimer), but some do evolve from the base forms that we have now. Those are:
Exeggute>Alolan Exeggutor. Grass/Dragon type, EASILY a favorite due to how rediculous it looks.
Pikachu>Alolan Raichu. Electric/Psychic, surfer dude that rides its own tail like a board.
Cubone>Alolan Marowak. Ghost/Fire type, probably the best Alolan form in the main series. If we ever get held items and Thick Club is released in a manner similar to what it is in the main series, this guy will be a MONSTER (thick club doubles the attack stat of a marowak)
As for the rest... We don't know. They're going to be here "in the coming weeks", and thats it.
If you want a full Alolan Pokemon list, you can find it, along with a main-series explanation, here:
https://m.bulbapedia.bulbagarden.net/wiki/Regional_variant
....I'm personally excited for Alolan Muk. Its typing is downright amazing: weak to nothing but ground. I'm hoping to see it as a good defender!