At this point, not much is known about this new Pokémon. The unnamed Pokémon seemed to appear from Liko's pendant, when a shield suddenly manifested around her when a wayward strike from Amethio's Ceruledge was about to hit her. From its appearance, we can make an educated guess that it may relate in some way to Terapagos, the mascot of the upcoming DLC for Pokémon Scarlet and Violet, The Indigo Disk. If so, this Pokémon might have a similar relationship to Terapagos as Phione does for Manaphy.
Presumably this Pokémon will continue to appear throughout Pokémon Horizons: The Series, so with any luck we should learn more about it soon.
UPDATE: A limited amount of additional information about this newly discovered Pokémon was revealed in a press release sent out Monday, including high resolution versions of the screenshots we originally featured in this announcement. From this update, we can now confirm that this Pokémon will be found in the upcoming The Hidden Treasures of Area Zero DLC for Pokémon Scarlet and Violet, and that it does have a relationship of some kind to Terapagos. No further information has been given at this stage.
Official description of the Pokémon from the Press Release said:This unknown Pokémon is reminiscent of the Legendary Pokémon Terapagos, but its name and true nature are shrouded in mystery. It is small in stature and not particularly strong, but when push comes to shove, it can crystallize the energy in its body to form a protective shield. Apparently, it can also assume a dormant state when it feels that its life is in danger by pulling its head, limbs, and tail into its shell and making itself look like a jewel.