Wynaut was introduced in the third generation of Pokémon as a pre-evolved form of Wobbuffet, and you will need to acquire one if you want to compile the national Pokédex of Pokémon Brilliant Diamond and Brilliant Pearl. However, this Pokémon cannot be caught in the wild in either Brilliant Diamond or Brilliant Pearl; the only way to get a Wynaut is to breed its evolved form, Wobbuffet.
Before you can breed for a Wynaut, you'll first need to unlock the National Pokédex, as neither Wynaut nor Wobbuffet are part of the Sinnoh Region Pokédex. Next, you'll need to acquire a Wobbuffet, which you can find using the PokéRadar in Lake Valor, Lake Verity, or Lake Acuity. You can also find one wandering around the hideouts of Dazzling Cave or Stargleam Cavern in the Grand Underground. We recommend that you check out the Grand Underground's hideouts, as you can also catch a Ditto there, which makes the whole breeding process a lot easier. Finally, you'll need the Lax Incense, which can be found on Route 225 behind a small tree that can be cut down.
After meeting the above prerequisites, equip your Wobbuffet with the Lax Incense, which allows the Pokémon to spawn Wynaut. Attempting to breed Wobbuffet without one holding this item will only produce more Wobbuffet. Now go to Solaceon Town Pokémon Nursery and leave your Wobbuffet with either a Wobbuffet of the opposite sex or a Ditto, which is able to breed with any Pokémon regardless of its gender. Eventually, the two Pokémon will produce an egg that will hatch a Wynaut. You can use the Egg Monitor Pokétch app to see when the egg is ready for pickup.