Twilight Phoenix, or "Different From Dark Phoenix?"

It has (NYI) attached to the name, which means Not Yet Implemented.  The item itself is labeled a "mount" but the name is [Essence of the Underworld] and the use effect is not your normal mount info:

Teaches you how to transform into the Twilight Phoenix, allowing you to fly and carry an ally with you. This is a very fast mount.

If I'm reading this correctly, this mount transforms the user into a Twilight Phoenix and allows someone else to ride on your back.  Considering that the Dark Phoenix is already implemented, could this be another phoenix model?  There's also the possibility that it's only an item that you use for a quest line and that it is not an actual mount.

Other confirmed and unconfirmed mounts that MMO-Champion data-mined:

• Goblin Trike Key (Horde)
• Goblin Turbo-Trike Key (Horde)
• Volcanic Stone Drake
• Phosphorescent Stone Drake
• Drake of the East Wind
• Drake of the West Wind
• Drake of the North Wind
• Drake of the South Wind
• Vitreous Stone Drake
• Brown Riding Camel (Ramkahen - Exalted)
• Tan Riding Camel (Ramkahen - Exalted)
• Grey Riding Camel (Ramkahen - Exalted)
• Skeletal Mount
• Blood Guard's Mount
• Crazy Raptor 75 (Bloodlord Mandokir's Raptor)
• Crazy Raptor 150 (Bloodlord Mandokir's Raptor)
• Black Dragonhawk
• Secret Goblin Raptor
• Fossilized Raptor
• Dark Phoenix (Guild Flying)
• Lion (Guild - Alliance)
• Thunder Lizard (Guild - Horde)

Other confirmed and unconfirmed pets data-mined:

• Pet Fish
• Pet Stone
• Turtle Egg (Albino)
• Turtle Egg (Hawksbill)
• Turtle Egg (Leatherback)
• Turtle Egg (Olive)
• Enchanted Jade
• Enchanted Purple Jade
• Celestial Dragon
• Crawling Hand
• Pebble
• Fossilized Hatchling
• Snail Pet (Guild)
• Baby Dark Phoenix (Guild)
• Hardboiled Egg

(source)