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>While an ordinary Ursaluna bears a yellow moon mark on its forehead, the one on this Ursaluna is red. This distinctive mark has earned it the name “Bloodmoon Beast,” and it is the only such Ursaluna known to exist. The Pokémon is unique in that its altered appearance cannot be attributed to Evolution or form change.
>It is said that a certain Ursaluna crossed the seas and arrived in the land of Kitakami—and that it then underwent a unique transformation over many years as a result of surviving in its harsh new environment. How many grueling years has this imposing Ursaluna endured?
Lore from the website
It's not an evolution or a form change, it was once a regular Ursaluna that mutated from living in Kitakami