Pokémon Legends: Arceus has added more than a few new Pokémon by providing older Pokémon new Hisuian regional forms or evolutions where they may have previously had none.
One Pokémon to receive a new evolution is the Johto native Ursaring, who can now evolve into the mighty Ursaluna when you meet the right requirements. The first requirement is locating an item called the Peat Block. This is a block of muddy material used to fuel fires, but because of its earthy nature, Ursaluna loves it. And when used on an Ursaring under the light of a Full Moon, you can evolve it into an Ursaluna yourself.
The issue is that Peat Blocks are hard to come by, as you will mostly need to rely on randomly-generated item spots within the Crimson Mirelands to find it. This is done by using the ridable Ursaluna provided to you earlier in the game as an Itemfinder to sniff it out.
After you obtain the item, you can use it on an Ursaring whenever there is a Full Moon in the night sky. This will trigger the evolution into its new Normal/Ground-type Hisuian evolution, which will make it an even more devastating force for your team.