The Zandalari Direhorn is a horde-only mount that is available in Battle for Azeroth. In order to get it, you need to complete the recruitment scenario for the Zandalari trolls. Upon completion you will unlock the Zandalari trolls as a playable Allied Race, as well as the Zandalari Direhorn mount. The mount can be used on all horde characters, and it's not tied to the Zandalari trolls.
To start the recruitment scenario, head to the Emissary Building in Orgrimmar and pick up the quest. There will most likely be a requirement to begin the scenario, but it's currently not known what the requirements are. This post will be updated once the requirements are revealed.