Green Qiraji Resonating Crystal is a rare mount summoning device.
This item drops in the Temple of Ahn'Qiraj from random mobs.
This mount, the Green Qiraji Battle Tank, grants 100% increased speed within the confines of the Temple of Ahn'Qiraj. They are fairly common, also coming in the red, blue, and yellow varieties.
There is another, more special mount that is accessed by completing the quest-chain to open the Gates of Ahn'Qiraj, and that is the Black Qiraji Battle Tank.