Received from a scientist in the same building as the person with a Patrat, if over 40 Pokémon have been seen in the Pokédex.
Source: Bulbapedia/ Castelia City
It's the same as in B/W, except now you have to have 40 Pokemon on your Pokedex in stead of 25.
And if you have a Pokemon Dream Radar
γ extension on Pokémon Dream Radar after capturing Landorus.
Source: Bulbapedia/ Dream Radar