Every event pokemon has a special ribbon (classic or whishing, it depends) which makes it untradable via gts and wondertrade... Also, except some rare cases, you can't nickname an event pokemon according that you're not his OT (original trainer, in this case XY&Z). Say this, I assume that your yveltals are certanly hacked/not legit. The case is closed.
" Sometimes you must HURT in order to KNOW, FALL in order to GROW, LOSE in order to GAIN because life's greatest lessons are learned through PAIN."
-Pain
