Completing the Pokedex is a whole lot harder in Pokemon X & Y. Back in the older versions, it is as simple as catch, breed and trade. In the latest X & Y, any Pokemon from the pre-gen that appears in you Pokedex will not be counted.
One fan known as benaclp brought this into light when he fall victim to this misunderstanding. Despite achieving all the certificates and the Oval charm, the Redditor’s failed to complete his Kalos Dex. This is all because of a Buizel from an older-gen which the lad received on GTS. Benaclp shared a screenshot of this incident.
The fix appears to be simple. After much investigation, the player revealed that catching the Kalos version of the Pokemon will solve the problem. After all, the Kalos Dex will only be complete if all the Pokemon were caught or hatched in Kalos itself.
The Pokedex in Pokemon X & Y can be really tough to complete. Previous Pokedex can be done solo. As for this latest game, it is highly advisable to have some friends helping you out. If not, then you can only wait for the Pokebank.