When you first visit the Pokémon Mansion in Brilliant Diamond and Shining Pearl, Mr. Backlot will tell you about his home's defining feature -- the Trophy Garden.
When you first discover the Trophy Garden as you collect the Sinnoh Gym Badges, there are only a few Pokémon you can catch.
List of Normal Pokémon in the Trophy Garden
Normal / Flying
Day: Very Common
Grass / Poison
How to unlock Daily Pokémon in the Trophy Garden
Naturally, you're not going to the Trophy Garden to catch Staravia and Roselia, but for the special Daily Pokémon.
To unlock them, you must have completed the Sinnoh Pokédex and unlocked the National Dex.
Then, speak to Mr. Backlot. He'll inform you that his Trophy Garden is now better than before.
Answer Yes to all of his questions, and afterwards he'll tell you which Pokémon are appearing daily.
List of Daily Pokémon in the Trophy Garden
There will be two Daily Pokémon active at any one time. A new Pokémon is added daily, with the older of the current two disappearing.
Here's what you can catch.
One thing to note when attempting to catch the Daily Pokémon is that they seem to operate on a different system to the normal Pokémon there, providing your lead Pokémon is exactly level 18.
Pop any Repel and run around in the grass -- you'll still encounter the Daily Pokémon or a Pikachu, but the others won't appear.
Use this trick to catch them more easily!
There are a total of 28 Unown to be found in the Solaceon Ruins in Pokémon Brilliant Diamond and Shining Pearl. Here's how to catch them all and unlock the Maniac Tunnel.