I believe this is a new Pokemon, maybe the one that could appear as the mystery specie teased in "Let's Go! Pikachu" and "Let's Go! Eevee"... If so, then that will prove those two games as compatible w/ Gen VIII in some way... Mythical Pokemon normally aren't announced until after the release of a new gen(until Magearna was announced 8 months prior to Gen VII's start, making it the first Mythical announcement to break this norm), so in the same vein as Magearna, since Gen VIII is not coming until next November, this Mythical can enjoy some free time before the new Mythical can be transferred to Gen VIII, at which point people can start capturing it, and for once, you could snag one Mythical at launch. Neat!
^ But if that is not a new pokemon, maybe this is Gen 8's MissingNo. sprite, glitch and all...