Flamewing is not a natural phenomenon. He was created by Arzaki some time after the latter's coming to BZ-Nui. During a hiking venture into the mountains with Ghiddy, Arzaki lost his footing and fell from a cliff. In his moments of panic, he unleashed the full extent of his Fire powers, which coalesced beneath him in the form of a large winged bird-rahi, formed of living flames. Arzaki, now on the back of the creature, flew back up to where the Matoran of Awesome waited, studying a dandelion he found whilst nibbling on a twig. Once back on solid ground, Arzaki drew the phoenix back into himself, but with great strain.
Upon his return to BZ-Metru, the Toa of Fire constructed an urn to contain the creature when not in use. He then released it again, and it entered the urn.
Not technically a vehicle, Flamewing is used as Arzaki's personal transport around the island of BZ-Nui.
The creature is formed of Arzaki's power, and is kept in a special burial urn when not in use. At times when the phoenix is weak or "hungry," Arzaki can feed it by channeling some of his power into the urn.
As a creature of pure flame, it is incredibly hot to the touch, and only a Toa of Fire or Plasma can withstand it. Due to this, Arzaki does not offer anyone else rides on the phoenix.
Over time, the fire creature began to develop minor sentience, modeling itself after the personality of its creator.
(To be added)