It’s no secret that the “Pokemon” franchise is inspired by the real world. Newer regions, such as Kalos and galar, are inspired by France and the United Kingdom respectively, but previous entries and their regions were based on Japanese geography. For example, sinno is based on Hokkaido, and since “Pokemon Legends: Arceus” is set in ancient times – Sinnoh, the game is rightly inspired by the pre-modern Japan of Hokkaido, especially its original inhabitants.
In “Pokemon Legends: Arceus”, Jubilife Village is the only sign of “modern” civilization in Hisui. The rest of the game world is covered in wild nature and mysterious ancient ruins. The only exceptions are the Diamond and Pearl clans, which predate Jubilife, and their camps. These clans are not so much an easter egg as a reference to real-life Ainu people.
Long ago, perhaps as far back as prehistoric times, the Ainu were the sole inhabitants of Hokkaido, reflecting all of the “Diamond and Pearl clans living in a land based on Hokkaido”. The Ainu relied on the Yezo sika deer for food and clothing, and the Diamond and Pearl Clans revere Kite, an evolution of Stantler supposedly based on the sika deer, and uses its fur to make warm winter clothes. Coincidence or intentional reference? How about the new Ursaring evolution, Ursalune? Not only is he a bear, but his new ground type and mountain-like body connect him to the Ainu god of mountains and bears, Kim-un-kamuy.
Maybe we read too much into things, but the Diamond and Pearl clans and several Hisuian Pokémon share too many ties with the Ainu to be a coincidence.