Ahzi Hare is Flickie's internal Vox. Lorewise, internal Vox were something Summoners could pull from people they have a close bond with, but when the magi waned dramatically from Nepheria, it became a thing of the past. Catt being part dragon (lung/wyrm to be specific), has a lot of inner magi and can tap into arcane practices.

Flickie actually transforms into the Vox before Catt actually knows she can summon it. He has mysterious powers himself that he's unaware of. And, these powers net him a close connection to his Soulspace, therefore, when his emotions surge, he has access to powerful abilities. Such as the Ahzi Hare.