In a new support article, Blizzard has informed the playerbase that Tusks of Mannoroth is currently bugged and only learnable by plate wearers. However, this is not intended and will be fixed in the future, allowing any character to learn the cosmetic.
Note that this doesn't mean non-plate users will be able to transmog it, just that you are able to collect the appearance with any character for your plate wearers to use it.
This issue is currently planned to be fixed in a future patch. While Customer Support cannot provide the release date of future patches, please keep an eye on our official blogs and social media channels for announcements from our developers.
Players can purchase Tusks of Mannoroth from Pythagorus in any Infinite Bazaar. Tusks of Mannoroth costs 20x Bones of Mannoroth and 0 Bronze.
You can obtain Bones of Mannoroth from Garrosh Hellscream in Siege of Orgrimmar on Normal or higher difficulties. The amount of bones per kill is as follows: