One might think Yanma would be easier to find in Pokémon Brilliant Diamond and Shining Pearl since it is one of several Pokémon that received a new evolution in the original fourth generation titles. However, the remakes have locked the poor Bug/Flying-type in the post-game for some inexplicable reason.
Essentially, if you wanted to use Yanma on your main team while working to become the Champion of Sinnoh, you are out of luck unless you trade with someone to get it earlier than it is naturally available.
If you don’t have anyone to trade with or don’t care about using it on your main team, you can pretty easily obtain it after becoming Champion, as it will be appearing frequently in one specific location later on.
After becoming Champion, you will need to set out and complete your Sinnoh Pokédex. That’s right. Even though Yanma’s evolved form Yanmega is native to Sinnoh, it isn’t included in the region’s Pokédex for some reason.
Once you unlock the National Pokédex, Yanma will become available as one of six different Daily Pokémon in the Great Marsh over in Pastoria City. You get a 10 percent chance of finding it as one of the rare spawns when using the binoculars in the observatory before you go in, but it isn’t a guarantee by any means.
Overall, your best bet might just be to find a friend who has a second Yanma sitting in their PC or may have gotten lucky and found a few when exploring the Great Marsh in the post-game. Otherwise, you will need a bit of luck to finally catch your Yanma even after completing part of the Pokédex.
And as for evolving Yanma into Yanmega, the process is pretty easy.
Once you have a Yanma, all you need to do is train it up to level 33 and teach it Ancient Power, which it naturally learns at that level. If you somehow end up with a Yanma that is higher level than that or forget to teach it Ancient Power, you can still teach it again by going to the Move Relearner just South of the Great Marsh in Pastoria.
If you do have to relearn Ancient Power, you will just need to have Yanma level up once after getting the move to trigger the evolution.
