Where should I start training to get a Gyarados with Flying STAB?

Question

I already have Y, and also want to get Sun/Ultra Moon. Gyarados is now able to learn Hurricane (and get Flying STAB) from Gen 7 onwards. Should I get the Magikarp/Gyarados in Y, then transfer it with Pokémon Bank to Sun/Ultra Moon when it's reach level 48 (where can learn Hurricane), or catch it in Sun/Ultra Moon and transfer to Y when it learned Hurricane?
(Edit: To be exact, the Gyarados will be transferred into my Y game)

Comments

Why would you want hurricane on Gyarados? Sure, it gets STAB, but Gyarados's special attack is so weak that physical non-STAB attacks usually do more damage.

Answer 1

You definitely want to catch the Magikarp or Gyarados in X/Y and then trade to S/M/US/UM once it learns Hurricane, as it is impossible to transfer Pokemon from Sun and Moon or Ultra Sun and Ultra Moon back to X and Y.

Comments

...except that Gyarados learns Hurricane at lv 48 only in Sun/Moon/Ultras, not XY