How did I know Alolan Rattata was gonna be dark type?..(LEAKS)
Just a reminder: that specific Alola form "leak" has been 0% correct so far. It claimed Rattata would be pure Dark-type, and that Ninetales would be Fire/Fairy-type. And the fact that we've seen normal Arcanine in game footage, while not completely ruling out Alolan Arcanine, makes it seem pretty unlikely.
Do you guys remember back when 3rd-gen came out, and all the 1st and 2nd-gen Pokémon got Abilities for the first time? GameFreak was incredibly lazy with some of those. Over half of the Fire-types got Flash Fire, over half the Electric-types got Static, and almost all of the Grass-types got Chlorophyll. A bunch of the Normal/Flying-types got Keen Eye. At the same time, a lot of Pokémon just have horrible abilities like Run Away. But for lots of these Pokémon, they already have two Abilities and a Hidden Ability, meaning no available slots for new Abilities.
Also, Ice, Ghost, Dragon, Dark, Steel, and Fairy-types are pretty rare among the original 151 Pokémon.
So what do we have so far?
• Exeggutor loses Chlorophyll and gains the Dragon type.
• Sandslash gains both the Ice and Steel types.
• Ninetales loses Flash Fire and gains the Ice and Fairy types.
• Meowth gains the Dark type.
• Marowak gains the Ghost type.
• Raichu loses Static.
• Rattata loses Run Away and gains the Dark type.
I expect most of the other Alola forms to fall into this same mold. Probably either Victreebel or Vileplume will get a new form, if not both. Rapidash and Dodrio have poor, common Abilities too.
But take all this with a block of salt. I would really have expected Fearow to get an Alola form, but we've already seen normal Fearow in footage, so I'm not optimistic there.