Rhaknam, an arcwhale.
Arcwhales are large airborne whales that roam the skies of Arcadia. Common archwhales are never actually seen in Skies of Arcadia.
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The only known arcwhale in the game is the purple Gigas, Rhaknam. However in the first encounter with Rhaknam Vyse and Aika recognize it as an arcwhale and even state they "have never seen an arcwhale that big before" implying the creature isn't that uncommon.