In Route 212, there is a Mansion where a few people live. Out back there is a patch of grass which is predominantly inhabited by Roselia, Mukubird, Pichu, Pikachu and Korotokku. However if you talk to the owner in the room to the right, you can activate something else.

Every day if you talk to him, he'll walk out. Go back in and he'll be there and tell you of a Pokémon that is inhabiting the place today. Sometimes its a Rare Shinou Dex Pokémon, others its a Non-Shinou Dex Pokémon so be alert. They are however there all day so you can go back. Their level averages at Lv. 16 to Lv. 19 so bring a Pokémon of that level. You have up to two of these Pokémon at once, they have a low encounter rate in the Mansion and the two Pokémon are the one allocated Today and Yesterday.