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Ark-FFIX
Ark (アーク, Āku?) is an airship-like summon that first appeared in Final Fantasy IX. Although initially appearing as an airship with a dragon's head at the helm and a long blade out front, Ark can transform into a mecha; in this state, it can attack with a heavy arsenal of explosive laser weaponry.
Ark seems to be Terra's equivalent to Gaia's Alexander their elements mirroring each other: Ark is Shadow-elemental and Alexander is Holy-elemental; both are mechanical in nature, being feared as ancient weapons and summons, with Alexander being an immobile megalithic fortress while Ark takes a form similar to an airship; they share the theme of Heaven and Earth, Ark being airborne and Alexander being earthbound; both represent their respective worlds of origin and their names relate to the a theme of judgment and punishment, with Alexander known for his attack, Divine Judgment, while Ark's original attack name in Japanese was Dark Fate.
The one people have seen in game including myself does seem to have some characteristics that match Ark such as being an airship and the long pointy part on the ship plus given how Alexander has just become part of the game it makes some sense to me Ark might also be added. The colour scheme also seems to match the idea of being shadow type eidolon in the purple/black appearance. So my bet is on Ark which might be the next raid added after Alexander (alexander being holy type and Ark being shadow type plus heaven/earth aspects).