Reins of the Emerald Pandaren Phoenix

The Reins of the Emerald Pandaren Phoenix is a Pandaren Phoenix mount achievement reward.

Source
When you complete the achievement Challenge Conqueror: Silver, you will receive one Ancestral Phoenix Egg in the mail. This can be redeemed for a Emerald Pandaren Phoenix mount. To redeem, visit located in the Vale of Eternal Blossoms, outside the Seat of Knowledge.