The Unshackled Waveray mount is a brand new Horde-only mount added in patch 8.2 of World of Warcraft. This is tied to the Nazjatar content, and only the Horde can get this. The Alliance counterpart is the Anokan Waveray.
To get the Unshackled Waveray, you will need to reach Exalted with The Unshackled. This is a new faction in 8.2, and you mainly get reputation by doing daily quests, world quests and other activities in Nazjatar. Once you reach exalted with them, you will be able to buy the mount for 250 x Prismatic Manapearls. These are also tied to pretty much the same activities that give you rep, but if you don't have them by the time your reach exalted, check out this guide to farming Prismatic Manapearls.
When you are Exalted and have the 250 manapearls, go to the quartermaster of The Unshackled, called Finder Pruc. He will sell you the mount, and that's all you need to do to get it. Enjoy your new waveray!