this is in game only, so hordes and friend safari only (Ratatta, for example which was available at the start in Pleasant Forest, I've noticed is un-obtainable in-game on X/Y, )
Click here and scroll down to 'Hidden Abilities through Transfer'.
All Pokemon on that list were available in Gen V, but are no longer available in Gen VI and needs PokeTransfer in order to be obtained.
Hope I helped. :)
Source: Above Link