You cannot legitimately obtain Dratini in Pokemon Reborn. By "legitimately" I mean that it cannot be obtained without the use of Mods such as Randomizers / Sandbox
The obtainable pokémon page still says that it's not in the game. Your pokédex may claim that it's somewhere in Ametrine Mountain, but that just means the event exists; you cannot yet access it. Probably because it's hidden behind a Rock Climb area or something like that.
