Bloodmoon Ursaluna is a special Pokémon you can currently only get in the Pokémon Scarlet and Violet: The Teal Mask DLC.
Shortly after arriving to Kitakami, you’ll see a woman named Perrin to the west of the school. She’s holding a camera and looks an awful lot like Adaman from Pokémon Legends: Arceus — which is where Ursaluna is originally from. She also has a Hisuian Growlithe!
Anyway, once you talk to her, she’ll task you with registering 150 Pokémon in your Kitakami Pokédex. If you were diligent and completed your Paldea Pokédex, you should already have 100 Pokémon registered in your Kitakami dex automatically. (That number may be 101 if you bothered to evolve Johtoan Wooper for Quagsire, despite it not being in the Paldea Pokédex.)
How to register 150 Kitakami Pokémon quickly in The Teal Mask
Catching 50 to 150 Pokémon can be daunting, but I was able to do so fairly quickly by just doing a lap around the area and hucking a Quick Ball at anything I didn’t already have. (I also lead my party with a False Swipe-having Zangoose, which also helps if the Quick Ball fails.)
You can quickly check to see if a Pokémon is already registered by targeting it with ZL. If you have it, it’ll have a Poké Ball by its name.
If you grow tired of catching Pokémon before you hit 150, you can use candy to level up your newly-caught Pokémon to evolve them for additional entries. Since the Pokémon in Kitakami start at around level 60 (if you’ve beaten the base game), you should only have to level a Pokémon once to evolve it (with a few exceptions).
How to catch Bloodmoon Ursaluna in Pokémon Scarlet and Violet
After you register 150 Kitakami Pokémon, you’ll have to battle Perrin. After beating her two Pokémon, she’ll tell you to meet her in the Timeless Woods. After snapping pictures for her and having a chat, you will go together to find the “Bloodmoon Beast.”
Unsurprisingly, the Bloodmoon Ursaluna functions a bit like a Titan when you battle it. It’ll have tons of HP and you won’t be able to catch it the way you do with regular Pokémon. Once you defeat it, the catch prompt will appear and you can use whatever Poké Ball you want to catch it guaranteed.
You cannot evolve Ursaring into a Bloodmoon Ursaluna. This is pretty much an event Pokémon and this is currently the only way to get it. They may add Peat Blocks (the item you use to evolve Ursaring into Ursaluna in Pokémon Legends: Arceus) down the line, but for now, you should savor and hold on to your rare bear.