You would have needed to transfer the Extremespeed Dratini obtained in HeartGold/SoulSilver to XY in order to even have it there. Otherwise there is no other way to obtain it. Wonder Trade usually is flooded with failed Dratini Eggs, and they usually carry Extremespeed.
You'll have to transfer the one from HGSS Dragon's Den, or another that's bred to have the move. I learned this on a question on here, but I forget the name. I think it was something like "How to get a ExtremeSpeed Dragonite with Mulitscale."
