The Blacksteel Battleboar mount is a reward unlocked by Guilds who have completed Guild Glory of the Draenor Raider, which is to clear Highmaul and Blackrock Foundry Mythic with at least 16 Guild players in the group.
Once your Guild has this unlocked, anyone who is Exalted rank with the guild can purchase the mount, regardless of if they helped with the Guild Achievement or not.
If you’re in a guild that has completed the mentioned achievement and you are Exalted reputation with your Guild, the next step is to locate a Guild Vendor. These are found in the majority of Horde and Alliance major cities and the mount can be purchased for 4,000g
If you aren’t Exalted with your Guild, then luckily it doesn’t take long to achieve, especially if you purchase the Renowned Tabard if available or even the Illustrious Tabard, as these will give you bonus rep. From here just complete quests and/or dailies and you will get a good chunk of reputation per quest you hand in.